🐘 PHP FAQs

PHP (Hypertext Preprocessor) is a server-side scripting language designed for web development. It runs on the server and generates dynamic HTML content. PHP can interact with databases, handle forms, manage sessions, and build full-scale web applications. It’s embedded within HTML and widely used in CMS platforms like WordPress, Joomla, and Drupal.

In PHP, variables are declared using the `$` symbol followed by the variable name. Example: ` = 'Raushan';`. Variable names must start with a letter or underscore and cannot contain spaces or special characters. PHP is loosely typed, so you don’t need to declare the data type explicitly.

PHP supports several data types: string (`'Hello'`), integer (`42`), float (`3.14`), boolean (`true/false`), array (`[1, 2, 3]`), object, NULL, and resource. You can use `gettype()` to check a variable’s type. PHP automatically converts between types when needed, but strict comparison (`===`) checks both value and type.

PHP supports single-line and multi-line comments. Use `//` or `#` for single-line comments, and `/* ... */` for multi-line. Example: `// This is a comment` `/* This is a multi-line comment */`. Comments help document code and are ignored during execution.

`echo` and `print` both output text to the browser. `echo` can take multiple parameters and is slightly faster. `print` returns a value (1), so it can be used in expressions. Example: `echo 'Hello';` `print 'World';`. For most use cases, `echo` is preferred due to performance and flexibility.

A function is a reusable block of code. Define it using `function` keyword: `function greet() { echo 'Hello ' . ; }`. Call it with `greet('Raushan');`. Functions can accept parameters, return values, and help organize code into logical units.

Use `mysqli_connect()` or PDO. Example: ` = mysqli_connect('localhost', 'user', 'pass', 'dbname');` Check connection with `if (!) { die('Connection failed'); }`. Always sanitize inputs and use prepared statements to prevent SQL injection.

Loops repeat code blocks. PHP supports `for`, `while`, `do...while`, and `foreach`. Example: `for ( = 0; < 5; ++) { echo ; }` Use `foreach` for arrays: `foreach ( as ) { echo ; }`.

All four functions are used to include external PHP files. `include()` and `require()` behave similarly, but `require()` throws a fatal error if the file is missing, halting execution. `include()` only throws a warning and continues. `include_once()` and `require_once()` ensure the file is included only once, preventing redeclaration errors. Use `require_once()` for critical files like configuration or database connections. For performance, avoid excessive use of `*_once()` in loops. Always validate file paths and use absolute paths when possible to avoid directory traversal issues.

PHP sessions are managed via `` superglobal and stored on the server, with a session ID passed via cookies. Start sessions using `session_start()` before any output. Common pitfalls include forgetting to call `session_start()`, session fixation attacks, and improper session timeout handling. Secure sessions by regenerating IDs (`session_regenerate_id()`), using HTTPS, and setting proper cookie flags (`HttpOnly`, `Secure`). For scalability, consider storing sessions in databases or Redis. Always validate session data before use to prevent injection or privilege escalation.

`==` checks for value equality after type juggling, while `===` checks for both value and type. For example, `'5' == 5` is true, but `'5' === 5` is false. Always prefer `===` for strict comparisons to avoid unexpected behavior, especially in conditionals and loops. Type coercion with `==` can lead to bugs when comparing strings, booleans, or nulls. Use `var_dump()` during debugging to inspect types and values. In security-sensitive logic (e.g., authentication), strict comparison is essential.

Use prepared statements with bound parameters via PDO or MySQLi. Avoid string concatenation in queries. Example with PDO: ` = ('SELECT * FROM users WHERE email = ?'); ([]);`. Validate and sanitize user input using `filter_var()` or custom logic. Never trust `Array`, `Array`, or `Array` directly. Use parameterized queries even for numeric values. For legacy code, audit all `mysql_query()` calls and replace them. Consider using ORM libraries like Eloquent or Doctrine for abstraction and safety.

Traits are a mechanism for code reuse in single inheritance languages like PHP. They allow you to include methods in multiple classes without inheritance. Define a trait using `trait Logger { public function log() { ... } }` and use it in a class via `use Logger;`. Traits are ideal for shared utility methods like logging, validation, or formatting. Avoid using traits for business logic or stateful behavior. If multiple traits conflict, resolve method precedence using `insteadof` and `as`. Traits improve modularity but can complicate debugging if overused.

Validate the file type and size using `Array`. Use `move_uploaded_file()` to save the file. Store uploads outside the web root, and sanitize file names to prevent path traversal.

`session_start()` initializes a session or resumes an existing one. It must be called before any output is sent to the browser to work properly.

Use `new PDO()` with your database credentials. Example: ` = new PDO('mysql:host=localhost;dbname=test', 'user', 'pass');`. Always use prepared statements to prevent SQL injection.

`echo` can output multiple values and is slightly faster. `print` returns 1, so it can be used in expressions. Both are used to display content.

Cookies are set with `setcookie()` and accessed using `Array`. You must call `setcookie()` before any output is sent to the browser.

Superglobals like `Array`, `Array`, ``, `Array`, etc., are built-in arrays in PHP that provide access to request and session data from anywhere in your script.

It suppresses error messages for expressions. Example: `@ = ;` will not produce a warning, but it's discouraged in production code.

Output buffering stores output in memory before sending it to the browser. Use `ob_start()` and `ob_get_clean()` to manipulate buffered output.

Use the `mail()` function for basic emails, or libraries like PHPMailer for advanced features like attachments and SMTP configuration.

Traits enable code reuse in classes. Define shared methods in a trait and include them with the `use` keyword in your classes. Useful for methods that apply across unrelated objects.

`==` compares values after type juggling, while `===` compares both value and type. Use `===` for strict comparisons to avoid unexpected behavior.

Use prepared statements with PDO or MySQLi. Avoid directly inserting user input into queries. Always validate and sanitize inputs.

`isset()` checks if a variable is set and not null. It’s commonly used to validate form inputs or session variables.

Use `define('CONSTANT_NAME', 'value');` or `const CONSTANT_NAME = 'value';`. Constants cannot be changed once set.

`explode()` splits a string into an array using a delimiter. Example: `explode(',', 'a,b,c')` returns `['a', 'b', 'c']`.

Use `header('Location: newpage.php');` followed by `exit;` to prevent further script execution.

`include_once` ensures the file is included only once, preventing redeclaration errors. Use it for configuration or shared functions.

Call `session_start()` at the beginning of your script before any output. Then use `` to store session data.

`empty()` checks if a variable is empty (null, false, 0, '', etc.). It’s useful for validating form fields or optional parameters.

Use `new mysqli('host', 'user', 'password', 'database');`. Always check for connection errors using ``.

`die()` stops script execution and optionally outputs a message. It’s often used for error handling or debugging.

Use `try-catch` blocks for exceptions, `error_reporting()` for error levels, and `set_error_handler()` for custom error handling.

An array is a data structure that stores multiple values. Use indexed arrays for lists and associative arrays for key-value pairs.

Use `foreach ( as => )` to iterate over arrays. It’s the most readable and efficient method for array traversal.

`substr()` extracts a portion of a string. Example: `substr('hello', 1, 3)` returns `'ell'`.

Use `filter_var(, FILTER_VALIDATE_EMAIL)` to check if an email is properly formatted.

`Array` retrieves data from the URL, while `Array` retrieves data from form submissions. Use `POST` for sensitive or large data.

Use `Array` to access file data, validate type and size, and move it using `move_uploaded_file()`.

`trim()` removes whitespace from the beginning and end of a string. Useful for cleaning user input.

Use `file_exists('filename.txt')` to check if a file is present before reading or writing.

`header()` sends raw HTTP headers. Use it for redirects, content type declarations, or cache control.

Use `class ClassName {}` syntax. Define properties and methods inside the class for object-oriented programming.

Inheritance allows a class to use properties and methods from another class using the `extends` keyword.

Use `__construct()` inside a class to initialize properties when an object is created.

Use `session_destroy()` to end the session and `unset()` to clear session variables.

`count()` returns the number of elements in an array. Useful for loops and validations.

Use `json_encode()` to convert arrays to JSON. Set the header with `header('Content-Type: application/json');`.

`array_merge()` combines two or more arrays into one. Keys from numeric arrays are reindexed.

Use `sort()` for indexed arrays and `asort()` or `ksort()` for associative arrays.

`strpos()` finds the position of the first occurrence of a substring. Returns `false` if not found.

Use nested arrays like ` = [['a', 'b'], ['c', 'd']]`. Access with `[1]`.

`array_keys()` returns all the keys from an array. Useful for iterating or checking existence.

Use `Array` or `Array` to retrieve form data. Validate and sanitize inputs before processing.

`htmlspecialchars()` converts special characters to HTML entities, preventing XSS attacks.

Use `function functionName() {}` syntax. Functions can return values using `return`.

`global` allows access to global variables inside functions. Use with caution to avoid side effects.

Use `is_array()` to confirm the variable type before performing array operations.

`implode()` joins array elements into a string using a delimiter. Example: `implode(',', ['a','b'])` returns `'a,b'`.

Use `try-catch` blocks. Catch specific exceptions and handle errors gracefully.

`require_once()` includes a file only once, preventing redeclaration errors. Use for critical files like configs.

Use `isset()` to check if a variable exists and is not null.

`is_numeric()` checks if a variable is a number or numeric string. Useful for input validation.

Use `date('Y-m-d')` or `DateTime` objects for flexible formatting and timezone handling.

`array_slice()` extracts a portion of an array. Useful for pagination or limiting results.

Use `rand(min, max)` or `mt_rand()` for better performance. For cryptographic use, prefer `random_int()`.

`file_get_contents()` reads a file into a string. Useful for reading text files or API responses.

Use `fopen()`, `fwrite()`, and `fclose()` to write data. Always check permissions and handle errors.

`unlink()` deletes a file from the server. Use with caution and validate paths before deletion.

`echo` can output multiple strings and is slightly faster. `print` returns 1 and can be used in expressions. Both are used to display output.

Use `define('NAME', 'value');` or `const NAME = 'value';`. Constants are global and cannot be changed once set.

`isset()` checks if a variable is set and not null. It's commonly used to validate form inputs or session variables.

Use `new PDO('mysql:host=localhost;dbname=test', 'user', 'pass');`. Always use prepared statements to prevent SQL injection.

`include` throws a warning if the file is missing, while `require` throws a fatal error and stops execution.

Call `session_start()` before any output. Then use `` to store session data.

`Array` retrieves data from form submissions, while `Array` retrieves data from URL parameters.

Use `Array` to access file data, validate type and size, and move it using `move_uploaded_file()`.

`trim()` removes whitespace from the beginning and end of a string. Useful for cleaning user input.

Use `file_exists('filename.txt')` to check if a file is present before reading or writing.

`header()` sends raw HTTP headers. Use it for redirects, content type declarations, or cache control.

Use `class ClassName {}` syntax. Define properties and methods inside the class for object-oriented programming.

Inheritance allows a class to use properties and methods from another class using the `extends` keyword.

Use `__construct()` inside a class to initialize properties when an object is created.

Use `session_destroy()` to end the session and `unset()` to clear session variables.

`count()` returns the number of elements in an array. Useful for loops and validations.

Use `json_encode()` to convert arrays to JSON. Set the header with `header('Content-Type: application/json');`.

`array_merge()` combines two or more arrays into one. Keys from numeric arrays are reindexed.

Use `sort()` for indexed arrays and `asort()` or `ksort()` for associative arrays.

`strpos()` finds the position of the first occurrence of a substring. Returns `false` if not found.

Use nested arrays like ` = [['a', 'b'], ['c', 'd']]`. Access with `[1]`.

`array_keys()` returns all the keys from an array. Useful for iterating or checking existence.

Use `Array` or `Array` to retrieve form data. Validate and sanitize inputs before processing.

`htmlspecialchars()` converts special characters to HTML entities, preventing XSS attacks.

Use `function functionName() {}` syntax. Functions can return values using `return`.

`global` allows access to global variables inside functions. Use with caution to avoid side effects.

Use `is_array()` to confirm the variable type before performing array operations.

`implode()` joins array elements into a string using a delimiter. Example: `implode(',', ['a','b'])` returns `'a,b'`.

Use `try-catch` blocks. Catch specific exceptions and handle errors gracefully.

`require_once()` includes a file only once, preventing redeclaration errors. Use for critical files like configs.

Use `isset()` to check if a variable exists and is not null.

`is_numeric()` checks if a variable is a number or numeric string. Useful for input validation.

Use `date('Y-m-d')` or `DateTime` objects for flexible formatting and timezone handling.

`array_slice()` extracts a portion of an array. Useful for pagination or limiting results.

Use `rand(min, max)` or `mt_rand()` for better performance. For cryptographic use, prefer `random_int()`.

`file_get_contents()` reads a file into a string. Useful for reading text files or API responses.

Use `fopen()`, `fwrite()`, and `fclose()` to write data. Always check permissions and handle errors.

`unlink()` deletes a file from the server. Use with caution and validate paths before deletion.

Traits allow code reuse in classes. Define shared methods in a trait and include them with the `use` keyword.

Use `namespace MyApp;` at the top of your file. Access classes with `use MyApp\ClassName;` or fully qualified names.

PSR-4 maps namespaces to file paths. Composer uses it to autoload classes based on their namespace and directory structure.

Composer manages dependencies. Use `composer.json` to define packages and `composer install` to install them.

`spl_autoload_register()` registers a custom function to autoload classes when they are used.

Static binding resolves method calls at compile time using `self::`, while dynamic binding uses `this->` and resolves at runtime.

Use routes to map HTTP methods to functions. Return JSON responses, validate input, and handle authentication.

Composer simplifies dependency management, autoloading, and package installation. It’s essential for scalable PHP projects.

Use Composer to define and install packages. Keep `composer.json` updated and use semantic versioning.

Closures are anonymous functions that can capture variables from the surrounding scope. Useful for callbacks and functional programming.

Use `json_encode()` to create JSON and `json_decode()` to parse it. Set headers for API responses.

`echo` and `print` output strings. `printf()` formats output using placeholders. Use `sprintf()` to store formatted strings.

Use file caching, APCu, Memcached, or Redis. Cache HTML, queries, or API responses to improve performance.

This usually occurs when your PHP code has a typo or missing characters like `;`, `}` or parentheses. For example, `echo 'Hello'` without a semicolon will throw a syntax error. Always check code structure, brackets, and quotes.

This error happens when you try to use a function that does not exist or has not been included. Example: calling `mysql_connect()` in PHP 7+ will throw this error because the function is removed. Solution: check function name, spelling, or include the required file.

A Notice indicates minor issues (like an undefined variable) but execution continues. A Warning signals something more serious (like including a missing file), but the script still runs. A Fatal error stops execution immediately (like calling an undefined function).

This error occurs if you send output (echo, whitespace, HTML) before calling functions that modify headers (like `header()` or `setcookie()`). To fix, remove unwanted whitespace or output before header calls, or enable output buffering with `ob_start()`.

Enable error reporting with `error_reporting(E_ALL); ini_set('display_errors', 1);`. For production, log errors instead of displaying them using `ini_set('log_errors', 1); ini_set('error_log', 'errors.log');`. Use `try-catch` for exception handling.

This is a generic server-side error. Common causes include syntax errors, incorrect file permissions, or missing PHP extensions. Check the server error log for details. Ensure files have proper permissions (644 for files, 755 for folders).

This happens when a script runs longer than the configured limit (default 30 seconds). Example: an infinite loop. Fix by optimizing code, breaking tasks into smaller steps, or increasing the limit with `set_time_limit(60);` in the script or in `php.ini`.

This means your script used more memory than allowed. Example: processing huge arrays or files. Fix: optimize memory usage, unset unused variables, or increase memory in `php.ini` with `memory_limit = 256M` or dynamically with `ini_set('memory_limit', '256M');`.

Best practices: always validate and sanitize inputs, use `isset()` and `empty()`, handle database connections with try-catch, enable error logging, test code in development before production, and keep PHP and libraries updated.

A parse error happens when PHP cannot understand your code due to syntax mistakes, such as missing semicolons, unmatched parentheses, or incorrect string quotes.

This usually means there is a missing semicolon, mismatched quotes, or a syntax mistake in your code. Check the line number and surrounding lines carefully.

It means you are trying to call a function that hasn’t been defined or included. Check function spelling or ensure the required file is included.

This happens when you try to use a variable that has not been assigned a value. Always initialize variables before use or check with `isset()`.

This happens when you try to access an array key that doesn’t exist. Fix it by checking with `isset($array['key'])` before using it.

Notice: minor issues, script continues. Warning: more serious but script continues. Fatal Error: script stops execution immediately.

This occurs when output (echo, whitespace, HTML) is sent before calling header functions. Remove extra spaces or use output buffering with `ob_start()`.

Make sure no output is sent before `header()` or `setcookie()`. Remove whitespace outside PHP tags or enable output buffering.

Your script runs longer than PHP’s execution limit (default 30 seconds). Optimize your code or increase limit with `set_time_limit()` or `php.ini`.

Your script used more memory than allowed. Optimize memory usage, unset unused variables, or increase memory with `ini_set('memory_limit', '256M');`.

This happens when a class is not loaded. Check namespace, spelling, or include/autoload configuration (Composer or `spl_autoload_register`).

This happens if you use a bare word instead of a string. Example: `echo Hello;` instead of `echo 'Hello';`.

Check the denominator before division. Example: `if($b != 0) { echo $a/$b; } else { echo 'Error: divide by zero'; }`.

This happens when you try to loop over something that is not an array or Traversable. Check with `is_array()` before using foreach.

It is a generic error. Causes include syntax errors, missing extensions, wrong file permissions, or fatal errors. Check server logs for details.

This happens when PHP errors are hidden. Enable error reporting with `ini_set('display_errors', 1); error_reporting(E_ALL);` to see the error.

Use `try-catch` for exceptions, configure `error_reporting()`, and log errors with `error_log()`. Avoid showing raw errors in production.

It means an exception was thrown but not caught. Wrap code in a `try { ... } catch(Exception $e) { ... }` block to handle it.

This happens when a file cannot be included. Check file path, permissions, and ensure the file exists.

This happens when you declare the same function multiple times. Fix by using `include_once` or `require_once`.

It’s a low-level crash often caused by buggy extensions or running out of memory. Update PHP or disable faulty extensions.

It means you passed something other than an array to `implode()`. Always validate inputs with `is_array()` before passing.

It happens when the regex pattern is missing proper delimiters like `/pattern/`. Fix by wrapping your regex correctly.

It means database credentials are wrong. Check hostname, username, password, and database name.

`mysql_*` functions are removed in PHP 7+. Use MySQLi or PDO instead.

It means you are using old functions no longer recommended. Update your code to use modern alternatives.

It means the file or URL doesn’t exist or isn’t accessible. Check file path, permissions, or allow_url_fopen settings.

This happens if output is sent before calling `session_start()`. Place `session_start()` at the top of the script.

Check if `session_status() !== PHP_SESSION_ACTIVE` before calling `session_start()` again.

It means you’re trying to access an array index that doesn’t exist. Example: accessing element 10 in a 5-element array.

This error happens when you try to echo an object directly. Convert with `json_encode()` or define a `__toString()` method.

It happens with deep or infinite recursion. Optimize recursion or use loops instead.

This happens when `header('Location: ...')` redirects in a loop. Check your redirect conditions.

It means `json_decode()` failed. Use `json_last_error()` to check the error and ensure JSON is properly formatted.

It means you didn’t pass enough parameters to a function. Always provide the required arguments.

You passed a function result directly to a reference function like `end()`. Fix by assigning to a variable first.

It means you’re trying to use a string like an array. Example: `$str['a']`. Make sure the variable is an array.

It means the class is missing or Composer autoload is not set up properly. Run `composer dump-autoload` and check namespaces.

This means PHP cannot read the file. Fix by setting proper file permissions (644 for files, 755 for folders).

It means you are trying to access a private method from outside the class. Use public methods or change visibility.

This happens if you forget quotes around strings. Example: `echo Hello;` should be `echo 'Hello';`.

It means you passed something other than an array. Validate with `is_array()` before calling `array_merge()`.

It usually means missing a closing bracket `}` or PHP tag `?>`. Check code structure carefully.

It means one of the arguments is not valid (not an array). Validate before calling `in_array()`.

Use `include_once` or `require_once` to prevent including the same class multiple times.

It means you are trying to access a property of something that isn’t an object. Check with `is_object()` before use.

It means you’re trying to loop over something that isn’t iterable. Check type with `is_array()` or `instanceof Traversable`.

This means you are trying to access an array element from a null variable. Ensure the variable is initialized as an array first.

Remove any whitespace or echo before `header()` or `session_start()`. Place headers before HTML output.

A parse error happens when PHP cannot understand your code due to syntax mistakes, such as missing semicolons, unmatched parentheses, or incorrect string quotes.

This usually means there is a missing semicolon, mismatched quotes, or a syntax mistake in your code. Check the line number and surrounding lines carefully.

It means you are trying to call a function that hasn’t been defined or included. Check function spelling or ensure the required file is included.

This happens when you try to use a variable that has not been assigned a value. Always initialize variables before use or check with isset().

This happens when you try to access an array key that doesn’t exist. Fix it by checking with isset($array['key']) before using it.

Notice: minor issues, script continues. Warning: more serious but script continues. Fatal Error: script stops execution immediately.

This occurs when output (echo, whitespace, HTML) is sent before calling header functions. Remove extra spaces or use output buffering with ob_start().

Make sure no output is sent before header() or setcookie(). Remove whitespace outside PHP tags or enable output buffering.

Your script runs longer than PHP’s execution limit (default 30 seconds). Optimize your code or increase limit with set_time_limit() or php.ini.

Your script used more memory than allowed. Optimize memory usage, unset unused variables, or increase memory with ini_set('memory_limit', '256M').

It occurs when PHP fails to understand your code due to syntax errors such as missing semicolons or mismatched brackets.

Check for missing semicolons, unmatched quotes, or incorrect syntax around the reported line.

You are calling a function that is not defined or not included in your script.

Because a variable was used before it was initialized.

It happens when accessing a non-existent array key.

Notice: minor issue, script continues. Warning: more serious but continues. Fatal: execution stops.

Output was sent before header() function. Remove whitespace/echo before headers.

Ensure no output is sent before header() or setcookie().

Script exceeded the allowed runtime (default 30s). Increase limit or optimize code.

Script used more memory than allocated. Optimize or raise memory_limit.

The class is not included or autoloaded. Check file paths or namespaces.

You forgot to quote a string, e.g., Hello instead of 'Hello'.

Check denominator before division.

You tried to loop over a non-array/non-iterable variable.

A generic error. Check logs for syntax errors, permissions, or misconfigurations.

Errors are hidden by settings. Enable error_reporting(E_ALL).

Use try/catch, error_reporting, and log errors with error_log().

An exception was thrown but not caught with try/catch.

File is missing or inaccessible. Verify file path and permissions.

Same function declared more than once. Use include_once or require_once.

Low-level crash often due to buggy extensions or memory corruption.

implode() expects an array. You passed another type.

Regex pattern missing proper delimiters like /pattern/.

Database credentials are incorrect.

mysql_* functions were removed in PHP 7. Use MySQLi or PDO.

You are using outdated functions. Replace with modern equivalents.

The file/URL is missing or permissions are incorrect.

Output sent before session_start(). Call it at the start.

Check session_status() before calling session_start().

Trying to access a non-existent array index.

You echoed an object. Convert it with json_encode() or __toString().

Occurs from infinite/deep recursion. Convert to loops or limit recursion.

header('Location') is looping infinitely.

Check JSON with json_last_error() and ensure it is valid.

Function was called without enough parameters.

You passed a function call result instead of a variable to a reference function.

Trying to use a string as an array.

Autoload failed. Run composer dump-autoload and check namespaces.

File permissions prevent access. Fix permissions.

Trying to call a private method outside its class.

Forgetting quotes around strings.

Non-array passed to array_merge().

Likely missing } or ?>.

Second parameter must be array.

Class was included multiple times. Use include_once.

You accessed a property on null or non-object.

foreach() expects array/Traversable.

You accessed an array index on a null variable.

Remove whitespace or echo before headers/session_start().

You tried to treat an object like an array. Use object properties instead.

strlen() only works on strings. Pass a valid string.

You passed a non-countable variable. Use is_array() check first.

You tried to call a method that does not exist in the class.

Trying to access a private property from outside the class.

Usually means you missed a closing bracket or semicolon.

Function received an argument of the wrong type.

PHP does not have permission to write to the file/directory.

Upload path is invalid or lacks permissions.

File path is wrong or missing.

explode() requires a string as the second argument.

You used an illegal type (like array/object) as an array key.

Your regex pattern is invalid.

You are calling a method without an object instance.

You misplaced if-statement or forgot braces/semicolon earlier.

Output was already sent before session_start().

All arguments to array_diff() must be arrays.

Second argument is not an array.

implode() requires an array as the first parameter.

The variable passed to foreach is not iterable.

You tried to access a constant that doesn't exist.

The interface was included multiple times.

You can only use isset() on variables, not function results.

You accessed an object with array syntax. Convert with (array) or json_decode(..., true).

All arguments must be arrays.

You passed a non-string value.

Your data structure has circular references.

You tried to define() a constant twice.

Trying to treat a string like an array.

Happens when overloading magic methods incorrectly.

You used self:: outside of a class.

You tried to manually call a class constructor.

trim() expects a string. You passed something else.

Only strings can be passed to htmlspecialchars().

basename() expects a string filename.

dirname() expects a string path.

The input string is not a valid date/time.

Second parameter must be a timestamp, not a string/array.

Default timezone is not configured. Use date_default_timezone_set().

Denominator is 0 in division. Check before dividing.

You attempted modulus operation with zero divisor.

Invalid math operation like sqrt(-1).

File not found or permission denied.

You passed an invalid resource.

The file handle is invalid or closed.

feof() only works with open file handles.

Both arrays must have equal size.

array_unique() only accepts arrays.

array_map() requires arrays as arguments.

You passed a non-array variable.

You passed a non-string value.

You used an unsupported encoding name.

Input string contains invalid multibyte characters.

You passed too short a string to pack().

Input string length too short for unpack().

Serialized string is corrupted or incomplete.

The serialized string is invalid or truncated.

number_format() requires numeric input.

bcdiv() cannot divide by zero.

Both arguments must be strings containing numbers.

Input must be a number or numeric string.

Your XML string is malformed.

The XML file cannot be opened.

File not found or inaccessible.

Schema file missing or invalid.

is_numeric() only works on scalar values.

filter_var() requires a string input.

The filter constant is invalid.

You passed an unsupported locale string.

Server restricted changing environment variables.

SMTP settings are misconfigured.

You passed an invalid recipient or headers.

The algorithm is invalid or unsupported.

You used an invalid algorithm name.

The cipher method is invalid or unsupported.

Wrong key, IV, or corrupted ciphertext.

password_hash() requires a string password.

First parameter must be a string.

Custom salt passed to password_hash() is not valid.

You passed zero or negative length.

The minimum is greater than the maximum.

Passed an unsupported timezone string.

Input string is not a valid date.

gmdate() expects timestamp as integer.

Invalid values for date/time arguments.

Arguments must be numeric and within valid ranges.

Fileinfo extension cannot find magic.mgc file.

The file does not contain EXIF data.

The JPEG file is missing or unreadable.

You passed an invalid GD image resource.

Image resource is already destroyed or invalid.

Font file path is invalid or unsupported.

Color value must be within valid range (0–255).

curl_init() expects a URL string or null.

The remote server closed the connection without response.

You passed an unsupported cURL option.

Invalid option keys in array.

You passed an invalid cURL handle.

The required database driver is not installed or enabled.

Database returned a general error, check SQL query.

You violated a database constraint such as unique or foreign key.

You referenced a non-existent column in SQL.

Your SQL query has a syntax error.

Bound parameter count does not match placeholders.

Database connection could not be established.

You executed an empty SQL string.

The query failed, result is invalid.

You passed an invalid query result.

Second argument must be a string.

The number of bound variables does not match placeholders.

Not all parameters were bound before execute().

The result set is empty or statement invalid.

You passed an invalid connection resource.

Execution failed due to invalid query or parameters.

ftp_connect() requires a hostname string.

Wrong username or password for FTP connection.

The local file cannot be opened for writing.

The local file cannot be read for upload.

The specified directory does not exist.

The file may not exist or permissions are wrong.

Invalid domain, type, or protocol specified.

The port is already in use or restricted.

Failed to listen on socket due to permissions or conflicts.

Socket could not accept due to error or closed connection.

Hostname or transport string invalid.

Port already in use or permission denied.

Passed resource is not a valid stream.

Resource is invalid or closed.

exec() requires a string command.

The shell command failed or is restricted.

System resources exhausted or permission denied.

System does not support process control functions.

PID or signal number is invalid.

The UID does not exist in the system.

Hostname is invalid.

The IP address cannot be resolved.

The DNS query failed due to invalid domain.

get_headers() expects a valid URL string.

The file is missing or not an image.

File not accessible or not a valid image.

You passed an invalid stream resource.

The filter name is already registered.

File path invalid or file not compressed.

The file handle is invalid or closed.

Handle is invalid or file is read-only.

The ZIP file is missing or corrupted.

The handle is invalid.

Phar archive is invalid or corrupted.

Phar extension is disabled or not allowed.

Your PHP bytecode is corrupted or incompatible.

OPcache memory exhausted. Increase opcache.memory_consumption.

Script consumed more memory than allowed in php.ini.

Script ran longer than max_execution_time setting.

It means you tried to access an array key that does not exist.

Because you declared a typed property but didn’t assign a value before using it.

A class with the same name was already declared or autoloaded.

The file path is incorrect or the file does not have the right permissions.

You passed a non-countable variable (like int or null) to count().

implode() only works with arrays, not strings or objects.

max() requires an array or multiple values. Passing other types causes this.

You are trying to write to a file handle that is not valid.

In PHP, isset() only works directly on variables, not expressions.

Files saved with UTF-8 BOM send output before headers. Save without BOM.

You passed an array or object instead of a string.

You tried to use an object in a numeric context.

You tried assigning to a function return value directly.

You tried to call a method on a variable that is null.

You declared the same variable twice in the same scope.

You tried to call a variable as a function but it’s not callable.

Your MySQL or PHP version doesn’t support utf8mb4.

strpos() requires a string haystack. Passing other types causes error.

PHP couldn’t save session data. Check session.save_path permissions.

You passed a wrong data type to filter_var().

In PHP 8, parent constructors are automatically invoked and cannot be called like a method.

It means the regex is too complex or invalid for PCRE.

You forgot quotes around a string, and PHP treated it as a constant.

You passed a function name that does not exist or is not callable.

Your query failed, so the result is invalid.

The PDO extension for that database is not installed or enabled.

Your PHP process lacks privileges to create raw sockets.

You accessed a string index that was never set.

You tried to access an array key on a null variable.

The GD library was compiled without JPEG support.

One of the arguments was not an array.

The function or method you tried to call does not exist.

You passed invalid XML content to simplexml_load_string().

Two different autoloaders loaded the same class twice.

PHP’s open_basedir prevents access to that directory.

The cURL extension is not enabled in PHP.

You tried to call a string variable as a function.

You accessed a protected class property from outside the class.

You called a non-static method with :: syntax.

You passed a non-object to get_class().

You used an unsupported or misspelled algorithm name.

You tried to use a namespace that doesn’t exist.

Both arrays must have equal length, otherwise error.

You declared the same interface more than once.

is_file() expects a valid path string. You passed another type.

You passed a non-string instead of a path.

You supplied an invalid or unsupported encoding.

Your regex consumed too much memory due to backtracking.

You passed a negative repeat count.

You passed an invalid type (like object or null) to str_replace().

You tried to unset() a character inside a string, which is not allowed.

You passed a non-array variable into array_merge().

You used an unsupported format character in pack().

extract() only works on arrays, not on strings or objects.

You defined the same function twice in your code.

You passed something other than an array to array_filter().

The XML string is incomplete or malformed.

Output (spaces, text, or BOM) was sent before calling header().

You tried to use [] on an integer value.

You passed the wrong type to mb_strlen().

The file pointer you passed is invalid.

reset() expects an array, but you passed something else.

The connection failed due to wrong host/port or firewall.

You attempted to declare the same constant twice in a trait.

The serialized string was corrupted or truncated.

You used a cipher that OpenSSL doesn’t support.

The FTP server is unreachable or blocked.

PHP’s open_basedir prevents checking that path.

You passed an invalid filter constant.

You accessed object properties with [] instead of ->.

Your regex is missing valid delimiters like /pattern/.

Your overridden method signature is different from parent class.

IMAP extension couldn’t connect. Wrong host/port or disabled SSL.

You passed non-string to strtoupper().

In PHP 8+, you cannot add strings using + operator.

array_search() requires array as haystack.

The fileinfo resource couldn’t be initialized.

Invalid username or password for FTP server.

You tried to chmod a socket, which is not supported.

The target class you tried to alias doesn’t exist.

Your data contains invalid UTF-8 sequences.

You passed an invalid or destroyed GD image resource.

The mbstring extension is not installed or enabled.

You declared the same static variable twice in one function.

You passed invalid data instead of resource array.

The local file doesn’t exist or remote path is invalid.

range() requires numeric start and end values.

The __clone() method is private or protected.

You used % with zero as divisor.

The hashing algorithm name is invalid or unsupported.

The condition you provided to assert() evaluated to false.

The file upload key does not exist in Array.

The temporary file could not be moved due to permissions.

You passed an empty string where a file path was expected.

You tried to seek in a non-seekable stream (like socket).

Class names are case-insensitive, so redeclaration fails.

The second argument must be an array, but wasn’t.

The GD library was not compiled with JPEG support.

Output buffering failed due to memory or configuration issues.

The image file is corrupted or inaccessible.

You passed an invalid or closed socket resource.

The system could not create a new process. Often caused by low resources.

You passed a variable name that doesn’t exist.

Two traits included in the same class declare the same method.

The object does not support assignment by reference.

Your hosting provider disabled process forking.

You called crypt() without specifying a valid salt.

The stream resource you passed is invalid or closed.

You tried to access a key that does not exist in the decoded array.

Wrong username/password or user lacks privileges in MySQL.

The FTP server did not return the size of the file.

The file cannot be opened due to path or permission issues.

You tried to echo or concatenate an array directly.

The function expected a closure, but you passed something else.

The input is not a valid array of arrays/objects.

The HTML string contains invalid entities.

The port is already in use or permission denied.

Your regex is missing a closing delimiter.

You referenced a class constant that does not exist.

The compressed data is corrupted or invalid.

The IMAP connection failed or was closed.

The cURL handle you passed was invalid.

Both arguments must be arrays, but one wasn’t.

You defined a constant with the same name twice.

Your INI file contains invalid characters or formatting.

The PNG file does not exist or cannot be read.

Your system lacks OpenSSL configuration or entropy.

PHP mail() cannot reach the configured SMTP server.

The target directory does not exist on FTP server.

You passed a directory where a file was expected.

You attempted to divide a number by zero.

You passed something other than array as second argument.

The image format is not recognized by PHP.

Another process is using the same IP/port.

You tried to save without specifying a valid file path.

The array must not be empty, but it was.

The executable path is wrong or missing permissions.

Your hosting provider disallows modifying execution time.

You passed an improperly formatted array of options.

Headers were already sent before starting session.

You passed an unsupported algorithm constant.

The remote file doesn’t exist or permissions denied.

The input was not an array.

The encoding you specified is not supported.

Invalid or empty data passed to gzencode().

You passed invalid image resources to imagecopyresampled().

You bound parameters incorrectly in your SQL statement.

You tried to register a wrapper that already exists.

Two traits in a class declared the same method without resolution.

This error occurs when you attempt to declare a function with the same name more than once in your code. PHP does not allow multiple definitions of the same function. This often happens if a file containing the function is included multiple times using include or require. To fix it, use include_once or require_once instead of include/require to prevent duplicate loading, or check your codebase for multiple function definitions with the same name.

The 'Undefined index' notice appears when you try to access an element of an array using a key that does not exist. For example, if you try to echo $arr['name'] but the 'name' key is not set in the array, PHP will show this error. To fix it, you should first check if the key exists using isset() or array_key_exists() before accessing the value. Alternatively, provide a default value using the null coalescing operator ?? in PHP 7+.

This error occurs when you attempt to modify HTTP headers (such as calling header(), session_start(), or setcookie()) after output has already been sent to the browser. Even a small whitespace or echo statement before header() can cause this issue. To fix it, ensure that all header modifications occur before any HTML output or echo statements. You can also use output buffering functions like ob_start() to delay sending output until headers are set.

This error happens because the old mysql_* functions (like mysql_connect, mysql_query) were removed in PHP 7. These functions are outdated and no longer supported. To fix the problem, you should update your code to use mysqli_* functions (like mysqli_connect) or use PDO (PHP Data Objects) for database interactions. Both options support modern features and provide better security, especially with prepared statements.

This error occurs when a PHP script takes longer to execute than the maximum allowed execution time set in the configuration. By default, PHP scripts have a 30-second execution limit. This often happens in long-running operations such as complex loops, heavy database queries, or file processing. To fix it, optimize your code, break tasks into smaller parts, or increase the execution time using set_time_limit() or by changing max_execution_time in php.ini.

This error means your PHP script attempted to use more memory than what is allowed by the memory_limit setting in php.ini. It usually happens when handling very large arrays, big file uploads, or inefficient recursive functions. To fix it, you can increase the memory limit using ini_set('memory_limit', '512M') or adjust php.ini. However, it's better to check your code for memory leaks or optimize your logic to use less memory whenever possible.

This error occurs when you try to call an object method on a variable that is null. For example, if you expect $obj to hold an object but it is actually null, calling $obj->method() will trigger this error. This often means that your object was not properly initialized or the function that should return it failed. To fix it, check your code to ensure that the variable is assigned a valid object before calling its methods.

This parse error indicates that PHP encountered unexpected text where it expected something else. It commonly occurs due to missing semicolons, incorrect use of quotes, or forgetting closing brackets. For example, writing echo 'Hello without semicolon will trigger this error. Carefully review the line mentioned in the error message and also the preceding lines, since the actual cause might be earlier than the reported line.

This error occurs when an exception is thrown in your code but not caught with a try...catch block. Exceptions are objects that represent runtime errors, such as database connection failures or file reading errors. If you do not handle the exception, PHP will terminate the script and display an error message. To fix it, wrap your code in try...catch blocks and implement proper exception handling strategies.

This notice appears when you try to use a variable that has not been declared or initialized yet. For example, echo $username will trigger this if $username has never been set. To fix it, ensure all variables are initialized before use, or check with isset() before accessing. Using strict error handling can help you identify such issues early in development.

This error happens when your code tries to divide a number by zero, which is mathematically undefined. For example, echo 5/0 will trigger this error. To fix it, always check the divisor before performing a division, and handle cases where it could be zero. You might provide a default value or display a user-friendly error message instead of crashing the script.

This error means that PHP could not find the class you are trying to use. Common causes include typos in the class name, missing include or require statements, or incorrect use of namespaces. To fix it, verify that your class file is being loaded correctly, use Composer’s autoload feature if available, and ensure the namespace matches your use statement.

This error occurs when you try to echo or concatenate an object directly. PHP cannot automatically convert objects into strings unless the class defines a __toString() method. To fix it, you can either implement __toString() in your class, or manually access the specific object property you want to display instead of printing the whole object.

This happens when you mistakenly try to treat a string like an array. For example, $str = 'hello'; echo $str['key']; will trigger this error because strings can only be accessed by numeric indexes. To fix it, make sure the variable you are using with array-like syntax is actually an array and not a string.

This error occurs when you attempt to call a function dynamically, but the variable used as the function name is not a string. For example, $func = 123; $func(); will trigger this error. To fix it, ensure that the variable holding your function name is a valid string, or use call_user_func() for safer dynamic function calls.

This error means that you have declared a class more than once in your code. It often occurs when the same class file is included multiple times using include or require. To fix it, replace include/require with include_once or require_once, or check your project structure to avoid duplicate declarations.

This error happens when a MySQL query fails, and mysqli_query() returns false instead of a result set. If you pass this false value to mysqli_fetch_assoc(), PHP complains. To fix it, always check if mysqli_query() was successful before fetching results, and use mysqli_error() to debug the query failure.

This error occurs when session_start() is called after output has already been sent to the browser. PHP needs to modify headers to start a session, but headers must be sent before any output. To fix it, call session_start() at the very top of your script, before any echo or HTML output.

This warning appears when you attempt to use foreach on a variable that is not an array or not iterable. For example, foreach(null as $item) will trigger this. To fix it, ensure the variable is actually an array or object before looping. Use is_array() or instanceof to validate the variable type before applying foreach.

This error occurs when you attempt to access an object property using array syntax. For example, $obj['property'] will fail unless the object implements ArrayAccess interface. To fix it, use the correct object syntax ($obj->property) or implement ArrayAccess in your class if array-like access is required.

This error occurs when you write a regular expression without proper delimiters. For example, preg_match('abc', $str) is invalid because 'abc' lacks delimiters. Correct usage would be preg_match('/abc/', $str). Always ensure that your regular expressions are wrapped with valid delimiters like /, #, or ~.

This error means that PHP could not find the file you are trying to include. Possible causes include incorrect file paths, missing files, or lack of permissions. To fix it, verify the file path, check your include_path settings, and ensure the file exists and is readable by the PHP process.

This error occurs when you try to call a method on an object, but that method does not exist in the class definition. It could be due to a typo, calling the wrong class, or forgetting to define the method. To fix it, check the class definition and ensure the method name is correct.

This error occurs when you attempt to define the same constant more than once using define(). Constants in PHP cannot be redefined once set. To fix it, use defined() to check if a constant already exists before defining it again. For example: if (!defined('MY_CONST')) { define('MY_CONST', 'value'); }

This error occurs when PHP does not have sufficient permissions to open the file for reading or writing. It usually happens when file permissions are too restrictive or when the PHP process runs under a user account without access rights. To fix it, adjust file permissions using chmod or change ownership with chown.

This error occurs when you attempt to declare a function with the same name more than once in your code. PHP does not allow multiple definitions of the same function. This often happens if a file containing the function is included multiple times using include or require. To fix it, use include_once or require_once instead of include/require to prevent duplicate loading, or check your codebase for multiple function definitions with the same name.

The 'Undefined index' notice appears when you try to access an element of an array using a key that does not exist. For example, if you try to echo $arr['name'] but the 'name' key is not set in the array, PHP will show this error. To fix it, you should first check if the key exists using isset() or array_key_exists() before accessing the value. Alternatively, provide a default value using the null coalescing operator ?? in PHP 7+.

This error occurs when you attempt to modify HTTP headers (such as calling header(), session_start(), or setcookie()) after output has already been sent to the browser. Even a small whitespace or echo statement before header() can cause this issue. To fix it, ensure that all header modifications occur before any HTML output or echo statements. You can also use output buffering functions like ob_start() to delay sending output until headers are set.

This error happens because the old mysql_* functions (like mysql_connect, mysql_query) were removed in PHP 7. These functions are outdated and no longer supported. To fix the problem, you should update your code to use mysqli_* functions (like mysqli_connect) or use PDO (PHP Data Objects) for database interactions. Both options support modern features and provide better security, especially with prepared statements.

This error occurs when a PHP script takes longer to execute than the maximum allowed execution time set in the configuration. By default, PHP scripts have a 30-second execution limit. This often happens in long-running operations such as complex loops, heavy database queries, or file processing. To fix it, optimize your code, break tasks into smaller parts, or increase the execution time using set_time_limit() or by changing max_execution_time in php.ini.

This error means your PHP script attempted to use more memory than what is allowed by the memory_limit setting in php.ini. It usually happens when handling very large arrays, big file uploads, or inefficient recursive functions. To fix it, you can increase the memory limit using ini_set('memory_limit', '512M') or adjust php.ini. However, it's better to check your code for memory leaks or optimize your logic to use less memory whenever possible.

This error occurs when you try to call an object method on a variable that is null. For example, if you expect $obj to hold an object but it is actually null, calling $obj->method() will trigger this error. This often means that your object was not properly initialized or the function that should return it failed. To fix it, check your code to ensure that the variable is assigned a valid object before calling its methods.

This parse error indicates that PHP encountered unexpected text where it expected something else. It commonly occurs due to missing semicolons, incorrect use of quotes, or forgetting closing brackets. For example, writing echo 'Hello without semicolon will trigger this error. Carefully review the line mentioned in the error message and also the preceding lines, since the actual cause might be earlier than the reported line.

This error occurs when an exception is thrown in your code but not caught with a try...catch block. Exceptions are objects that represent runtime errors, such as database connection failures or file reading errors. If you do not handle the exception, PHP will terminate the script and display an error message. To fix it, wrap your code in try...catch blocks and implement proper exception handling strategies.

This notice appears when you try to use a variable that has not been declared or initialized yet. For example, echo $username will trigger this if $username has never been set. To fix it, ensure all variables are initialized before use, or check with isset() before accessing. Using strict error handling can help you identify such issues early in development.

This error happens when your code tries to divide a number by zero, which is mathematically undefined. For example, echo 5/0 will trigger this error. To fix it, always check the divisor before performing a division, and handle cases where it could be zero. You might provide a default value or display a user-friendly error message instead of crashing the script.

This error means that PHP could not find the class you are trying to use. Common causes include typos in the class name, missing include or require statements, or incorrect use of namespaces. To fix it, verify that your class file is being loaded correctly, use Composer’s autoload feature if available, and ensure the namespace matches your use statement.

This error occurs when you try to echo or concatenate an object directly. PHP cannot automatically convert objects into strings unless the class defines a __toString() method. To fix it, you can either implement __toString() in your class, or manually access the specific object property you want to display instead of printing the whole object.

This happens when you mistakenly try to treat a string like an array. For example, $str = 'hello'; echo $str['key']; will trigger this error because strings can only be accessed by numeric indexes. To fix it, make sure the variable you are using with array-like syntax is actually an array and not a string.

This error occurs when you attempt to call a function dynamically, but the variable used as the function name is not a string. For example, $func = 123; $func(); will trigger this error. To fix it, ensure that the variable holding your function name is a valid string, or use call_user_func() for safer dynamic function calls.

This error means that you have declared a class more than once in your code. It often occurs when the same class file is included multiple times using include or require. To fix it, replace include/require with include_once or require_once, or check your project structure to avoid duplicate declarations.

This error happens when a MySQL query fails, and mysqli_query() returns false instead of a result set. If you pass this false value to mysqli_fetch_assoc(), PHP complains. To fix it, always check if mysqli_query() was successful before fetching results, and use mysqli_error() to debug the query failure.

This error occurs when session_start() is called after output has already been sent to the browser. PHP needs to modify headers to start a session, but headers must be sent before any output. To fix it, call session_start() at the very top of your script, before any echo or HTML output.

This warning appears when you attempt to use foreach on a variable that is not an array or not iterable. For example, foreach(null as $item) will trigger this. To fix it, ensure the variable is actually an array or object before looping. Use is_array() or instanceof to validate the variable type before applying foreach.

This error occurs when you attempt to access an object property using array syntax. For example, $obj['property'] will fail unless the object implements ArrayAccess interface. To fix it, use the correct object syntax ($obj->property) or implement ArrayAccess in your class if array-like access is required.

This error occurs when you write a regular expression without proper delimiters. For example, preg_match('abc', $str) is invalid because 'abc' lacks delimiters. Correct usage would be preg_match('/abc/', $str). Always ensure that your regular expressions are wrapped with valid delimiters like /, #, or ~.

This error means that PHP could not find the file you are trying to include. Possible causes include incorrect file paths, missing files, or lack of permissions. To fix it, verify the file path, check your include_path settings, and ensure the file exists and is readable by the PHP process.

This error occurs when you try to call a method on an object, but that method does not exist in the class definition. It could be due to a typo, calling the wrong class, or forgetting to define the method. To fix it, check the class definition and ensure the method name is correct.

This error occurs when you attempt to define the same constant more than once using define(). Constants in PHP cannot be redefined once set. To fix it, use defined() to check if a constant already exists before defining it again. For example: if (!defined('MY_CONST')) { define('MY_CONST', 'value'); }

This error occurs when PHP does not have sufficient permissions to open the file for reading or writing. It usually happens when file permissions are too restrictive or when the PHP process runs under a user account without access rights. To fix it, adjust file permissions using chmod or change ownership with chown.

This warning appears when PHP thinks you are referring to a constant but you actually intended to use a string. For example, writing echo name; without quotes makes PHP think 'name' is a constant. To fix it, always wrap string values in quotes, like echo 'name';. If you are truly using constants, make sure they are defined before use.

This is one of the most common PHP errors. It occurs when you try to send headers, cookies, or start a session after some output has already been sent to the browser. Even whitespace before

This happens when you call a non-static method using the ClassName::method() syntax. PHP distinguishes between static and instance methods. Static methods belong to the class, while non-static methods require an object instance. To fix it, either create an object before calling the method, or explicitly declare the method as static if appropriate.

This warning means you are passing a value directly into a function that expects a variable reference. For example, end(explode(',', 'a,b')) will trigger it because explode() returns a value, not a variable. To fix it, first assign the return value to a variable, then pass that variable to the function.

In PHP, you cannot declare a class inside another class or function. If you try to define a class inside a method or within another class definition, PHP will throw this error. To fix it, move your class declaration outside of functions or other classes so that it stands on its own.

This error occurs when you try to modify the return value of a function directly. For example, strlen('test') = 5 is invalid because strlen returns a value, not a variable. To fix it, store the function result in a variable and then work with that variable instead.

This usually means PHP reached the end of the file but was expecting more code. Common causes include unclosed braces, missing quotes, or incomplete statements. To fix it, carefully check your opening and closing brackets, parentheses, and quotation marks. IDEs or linters can also help you detect missing closures quickly.

This error occurs when you use a trait in a class but attempt to call a method that the trait does not actually define. It could also happen if there’s a naming conflict between multiple traits. To fix it, check the trait methods and ensure the method name exists or resolve conflicts using insteadof and as operators.

This error means you are trying to access a property on an object that has not been declared in the class. It could be a simple typo, or you may have forgotten to declare the property. To fix it, make sure the property exists in your class definition and is properly initialized.

This error occurs when you call a function or method without providing all required parameters. For example, function test($a, $b) requires two arguments, but calling test(1) will trigger the error. To fix it, provide all mandatory parameters or define default values in the function signature.

This happens when you pass more arguments to a function than it is defined to accept. For example, function add($a, $b) cannot handle add(1,2,3). To fix it, either remove the extra arguments or modify the function definition to accept them, possibly using variable-length argument lists (...$args).

This error happens when you try to access an array key on a variable that is actually a string. For example, $str = 'hello'; echo $str['key']; will trigger it. To fix it, make sure your variable is an array before accessing it with array syntax, or adjust your logic to use string functions instead.

This error occurs when you attempt to access a class property marked private from outside the class. Private properties are only accessible inside the class itself. To fix it, either change the property’s visibility to public or protected if external access is required, or use getter and setter methods to access it safely.

This error appears when you try to modify a property of an object that implements ArrayAccess, but PHP cannot directly change it that way. To fix it, you may need to reassign the value instead of trying to modify it in place. Always consult how ArrayAccess methods are defined in the class.

This means PHP could not find the file you are trying to include with require_once. Unlike include, require will produce a fatal error if the file is missing. To fix it, verify the file path, check permissions, and confirm the file exists in the expected location. You may also adjust include_path in your PHP configuration.

This error occurs when you try to declare the same interface multiple times in your code. It often happens when a file is included multiple times. To fix it, ensure you are using require_once or include_once, or structure your project to prevent duplicate interface declarations.

This error occurs when a class claims to implement an interface but does not actually define all required methods. Interfaces define a contract that classes must follow. To fix it, make sure your class has all methods defined exactly as required by the interface signature, including parameter types.

This error occurs when you try to clone an object of a class that has disabled cloning. In PHP, you can control cloning using the __clone method and by declaring private clone constructors. To fix it, check if cloning is allowed in the class, or create a custom clone method if necessary.

This error occurs when a class extends an abstract class but fails to implement all abstract methods. Abstract methods define required functionality but have no implementation. To fix it, either implement all abstract methods in the child class or declare the child class itself as abstract.

This happens when the same function name is declared multiple times. It could be due to including the same file twice or accidentally redeclaring the function in different files. To fix it, use include_once or require_once to avoid duplicate inclusions, and ensure your functions are uniquely named across the codebase.

This error often comes from SQL queries where the table or object name is invalid or misspelled. It could also mean the table does not exist in the connected database. To fix it, check your SQL query carefully, ensure table names are correct, and verify the database connection.

This happens when you attempt to use a PDO database connection but the required driver is not installed or enabled. For example, using mysql:host=localhost without having pdo_mysql installed will cause this error. To fix it, install the missing PDO driver (such as php-mysql), and enable it in your php.ini configuration.

This warning appears when you attempt to destroy a session without starting one first. PHP needs session_start() before you can manage sessions. To fix it, always call session_start() before session_destroy(), and check if a session is active using session_status() before destroying it.

This error occurs when you try to call a class method marked as private from outside the class. Private methods are only accessible within the class itself. To fix it, either change the method visibility to public or protected if outside access is needed, or restructure your code so that only internal calls use the private method.

This error occurs when PHP cannot locate the file you are trying to require. Unlike include, require will cause a fatal error if the file is missing. To fix it, ensure the file path is correct, check relative vs absolute paths, and verify that the file actually exists on the server.

This happens when you declare a class with a name that has already been used. It can be caused by duplicate includes or conflicting libraries. To fix it, rename your class to something unique, or ensure that files are only included once using require_once.

This warning occurs when you pass a non-array value to implode(). The implode function expects an array as its second argument. To fix it, check that the variable you are passing is an array, and cast it if necessary. Example: implode(',', (array)$variable).

This happens when your regular expression syntax is invalid. For example, unmatched brackets or missing delimiters will cause compilation errors. To fix it, carefully check your regular expression for typos, ensure proper delimiters are used, and test your regex with a tool before applying it in PHP.

This error appears when you pass something other than arrays to array_merge(). For example, array_merge('string', [1,2]) will fail. To fix it, make sure all arguments are arrays. If you are unsure, cast variables to arrays before merging, like array_merge((array)$var1, (array)$var2).

This error occurs when the string passed to json_decode() is not valid JSON. Common mistakes include single quotes instead of double quotes, trailing commas, or malformed syntax. To fix it, validate your JSON string and ensure it conforms to the proper JSON format. You can also use json_last_error() for debugging.

This warning means you are trying to access an array key on a variable that is null. For example, $data['key'] when $data is null will cause it. To fix it, check that the variable is an array before accessing it, and use null coalescing operator (??) to provide defaults safely.

This happens because the query failed and mysql_query returned false. Passing false into mysql_fetch_array() causes this warning. To fix it, always check the return value of mysql_query before fetching results, and use mysql_error() to debug why the query failed. Better yet, upgrade to mysqli or PDO since mysql_* is deprecated.

This warning appears when PHP assumes a bareword is a constant but no such constant exists. For example, echo HELLO; without defining HELLO will trigger it. To fix it, use quotes for strings, like echo 'HELLO'; or define the constant first using define().

This error means you passed a non-array value into array_filter(). For example, array_filter('string') will fail because it expects an array. To fix it, always pass an array, and cast variables if necessary using (array). Example: array_filter((array)$variable).

This error means the JSON extension is not enabled in your PHP installation. json_encode and json_decode are part of the JSON extension, which may not be compiled by default. To fix it, enable the JSON extension in php.ini or reinstall PHP with JSON support.

This happens when array_map() receives something other than an array for one of its arguments. For example, array_map('trim', 'string') will fail. To fix it, ensure all non-callback arguments are arrays. You can cast variables into arrays if needed, like array_map('trim', (array)$var).

This error occurs when some output has already been sent to the browser before header-related functions are called. Even a single whitespace or BOM at the top of the file can trigger it. To fix it, ensure no output occurs before headers, and check for hidden whitespace in included files.

This warning means the second parameter of in_array() is not an array. For example, in_array('a', 'string') will cause it. To fix it, check that the haystack argument is always an array, and cast if necessary, like in_array('a', (array)$haystack).

This error happens when the second parameter of array_key_exists is not an array. For example, array_key_exists('key', 'string') will fail. To fix it, make sure you pass an array as the second parameter, or validate the variable before calling the function.

This fatal error means you have defined the same function multiple times. It often happens when files are included multiple times. To fix it, ensure function names are unique, and use include_once or require_once to prevent duplicate inclusion of files.

This occurs when you try to start a session after output has already been sent to the browser. PHP requires headers to be sent before starting a session. To fix it, call session_start() at the very beginning of your script, before any HTML or echo statements are executed.

This error means one or both arguments to array_combine() are not arrays, or they do not have the same number of elements. To fix it, make sure both arguments are arrays of equal length before passing them to array_combine.

This happens when array_reduce() receives something other than an array as its input. For example, array_reduce('string', 'callback') will fail. To fix it, ensure the first argument is always an array. If you are unsure, cast the variable with (array) before passing it.

This error occurs when you try to access a constant from a class that has not been defined. For example, using MyClass::MY_CONST when MY_CONST does not exist will trigger this. To fix it, verify the constant name in the class definition. If it should exist, define it using the const keyword, or double-check that you are referencing the correct class and namespace.

This error occurs when you try to use the GD library functions in PHP but the GD extension is not enabled. By default, not all PHP installations have GD activated. To fix it, enable the GD extension in your php.ini file by uncommenting extension=gd or installing the extension via your package manager. Once enabled, restart your web server for changes to take effect.

This error means PHP tried to allocate more memory than the configured memory_limit. It happens often when processing very large files, arrays, or database results. To fix it, you can increase memory_limit in php.ini or via ini_set('memory_limit', '512M'). But the better approach is to optimize your code, process smaller data chunks, or use streaming techniques to avoid loading huge datasets all at once.

This occurs in PHP 7+ when strict typing is enforced and a function parameter receives the wrong type. For example, a function requiring an int but receiving a string triggers this. To fix it, either pass the correct type, cast the variable (int)$var, or remove strict type enforcement if appropriate. Type declarations help catch bugs early, so adjusting input values is usually best.

This error occurs when you try to create an instance of an interface directly using new. Interfaces define methods that must be implemented in a class but cannot be instantiated themselves. To fix it, instantiate a class that implements the interface instead of the interface itself. Ensure that your design pattern uses interfaces correctly for abstraction and dependency injection.

This error occurs when you try to dynamically create a class instance using new with an invalid class name. For example, $className = 123; new $className; will trigger this because the class name is not a string. To fix it, make sure the variable holds a valid class name as a string and that the class exists in your code or autoloader.

This warning occurs when json_encode() encounters invalid UTF-8 characters in your data. PHP expects all strings to be valid UTF-8 for JSON conversion. To fix it, ensure your strings are properly encoded. You can use mb_convert_encoding($string, 'UTF-8', 'auto') to clean up data, or pass the JSON_INVALID_UTF8_IGNORE flag to ignore invalid sequences. Handling character sets properly will prevent broken JSON output.

This notice occurs when PHP encounters a bare word that it interprets as a constant but which has not been defined. For example, echo hello; will trigger this, as PHP thinks hello is a constant. To fix it, enclose strings in quotes ('hello'), or check that the constant is actually defined. In PHP 7+, this notice was made stricter to avoid accidental behavior.

This warning appears when you pass a function result directly into a function that requires a reference. For example, end(explode(',', 'a,b,c')); will trigger this because explode() is not a variable. To fix it, assign the result to a variable first before passing it by reference. This ensures compatibility with functions expecting references like reset(), end(), or sort().

This error means you are trying to access an object as if it were an array. For example, $obj->property is correct, but $obj['property'] will fail unless the object implements ArrayAccess. To fix it, use object syntax (->) when dealing with objects, or cast the object to an array if you need array access. Be mindful of how data structures are returned from APIs or json_decode().

This error occurs when you try to use cURL functions without having the cURL extension enabled in PHP. To fix it, enable the extension in your php.ini file (extension=curl) and restart your web server. On Linux servers, you might need to install php-curl via your package manager. Without this extension, PHP cannot make HTTP requests via cURL.

This error occurs if you try to declare a class inside another class directly, which is not allowed in PHP. You cannot nest one class declaration within another. Instead, you should declare classes separately and then use them together through composition or inheritance. If you intended to create a property with a class type, declare it properly outside and reference it instead.

This warning means your regular expression is invalid. Common causes include unmatched parentheses, incorrect escape sequences, or unsupported regex tokens. To fix it, double-check your regex syntax. For example, preg_match('/(abc/', 'text') will fail because the parentheses are unbalanced. Using online regex testers can help you debug before applying in PHP.

This error occurs when you define the same function name more than once. It usually happens if a file is included multiple times. To fix it, use include_once or require_once instead of include or require. Alternatively, check your code for duplicate function declarations and remove or rename them. This ensures PHP loads each function definition only once.

This error occurs when you try to append to a string using the array [] operator. For example, $str[] = 'a'; is invalid because strings cannot be used with array append syntax. To fix it, use concatenation: $str .= 'a'; or explicitly convert your variable into an array before using [].

This error occurs when you try to use isset() directly on a function call result. For example, isset(myFunction()) is invalid. PHP requires a variable for isset(). To fix it, assign the function result to a variable first, then apply isset(). Example: $result = myFunction(); if (isset($result)) {...}.

This error occurs when you try to use an arithmetic operator (+, -, *, /) on incompatible types. For example, adding an array and a string triggers this. To fix it, make sure both operands are numeric or cast them properly. PHP’s type juggling can sometimes hide these issues, but in strict typing, they must be corrected explicitly.

This error occurs when you try to access a class property marked as private from outside the class. Private properties are only accessible within the class itself. To fix it, either change the property visibility to public or protected if external access is needed, or provide getter and setter methods to allow controlled access.

This happens when you try to clone an object that explicitly prevents cloning by declaring a private or final __clone() method. PHP does not allow cloning such objects. To fix it, remove or adjust the __clone() restriction if cloning is intended. Otherwise, design your application to avoid cloning that specific object.

This error means you are trying to unset a character in a string as if it were an array element. For example, unset($str[0]); will fail. Strings are not arrays in PHP, even though they can be accessed like arrays. To fix it, manipulate strings using string functions such as substr() or str_replace() instead.

This error happens when you try to define an array with an empty key, such as [ => 'value' ]. PHP does not allow array keys without a value. To fix it, either specify a key (like 'key' => 'value') or allow PHP to auto-generate numeric keys by just writing ['value'].

This error occurs when you use self:: outside of a class context. The self keyword refers to the current class, so it cannot be used in a non-class scope. To fix it, use the actual class name instead of self if you are outside the class. Inside the class, self:: works correctly to reference static properties and methods.

This happens when you attempt to read from an array using empty square brackets. For example, $value = $arr[]; is invalid because [] is only allowed for appending values. To fix it, specify a valid key or use functions like end() to get the last value.

This error occurs if you declare a variable multiple times within the same block using incompatible syntax. For example, if let-style declarations existed, you would see this. In PHP, this often occurs when a variable shadows itself inside loops or functions. To fix it, rename variables or avoid overwriting them unintentionally.

This error occurs when you try to call a class constructor explicitly as a method. For example, $obj->__construct() is not allowed. Constructors are automatically called when you create a new instance using new. To fix it, remove the explicit constructor call and instead use new MyClass().

This error occurs when you try to call an object method using Class::method() syntax without declaring it static. Non-static methods require an instance of the class. To fix it, either declare the method as static or instantiate the class before calling the method.

This happens in PHP 7.4+ when you declare a typed property but try to access it before assigning a value. For example, public int $id; echo $this->id; will fail because $id has no value yet. To fix it, initialize typed properties in the constructor or provide a default value.

This error occurs when you mistakenly attempt to add a string to an array using +. In PHP, arrays and strings cannot be combined this way. To fix it, ensure both operands are of the same type or use array_merge() for arrays and concatenation (.) for strings.

This occurs when you try to treat an array as a string. For example, echo $arr will fail because arrays cannot be directly converted to strings. To fix it, use print_r($arr) or var_export($arr) for debugging, or access specific elements by key instead of echoing the whole array.

This happens when you attempt to concatenate or interpolate an array directly into a string. For example, echo "My data: $arr" will fail. To fix it, use implode() to join array elements into a string, or loop through the array to build your output manually.

This error occurs when you try to unserialize a string that is corrupted or was not generated by serialize(). It often happens when data gets truncated during storage or transmission. To fix it, ensure the data being unserialized is intact and was originally created by PHP’s serialize() function. If you are handling external input, validate it carefully to avoid security risks.

This error occurs when you try to assign by reference to an object that overloads property access using __get() or __set(). PHP does not allow assigning by reference in this case. To fix it, remove the reference operator (&) and assign normally, or redesign the class to allow reference behavior explicitly.

This error happens when you try to use multibyte string functions without enabling the mbstring extension. To fix it, enable mbstring in php.ini (extension=mbstring) or install the appropriate package for your operating system. Without mbstring, PHP cannot handle multibyte character encodings properly.

This error occurs when you attempt to echo or concatenate an object without defining a __toString() method in the class. PHP does not know how to convert objects into strings by default. To fix it, implement a __toString() method in your class that returns a string representation, or extract the specific property you want to display.

This happens in PHP 8.1+ when you declare a property with the readonly modifier and later try to change its value. Readonly properties can only be set once at construction. To fix it, remove the readonly modifier if mutability is required, or design your class to avoid modifying such properties.

This error occurs when you pass a non-iterable variable (like an integer or string) into foreach(). PHP requires an array or Traversable object for foreach. To fix it, check the variable type before using foreach. Use is_iterable() in PHP 7.1+ to validate before looping.

This happens when you call a function with fewer arguments than it requires. For example, function add($a, $b) requires two parameters, but calling add(1) will trigger this error. To fix it, provide all required arguments or define default values in your function signature for optional parameters.

This occurs when you call a function with more arguments than it expects. For example, function greet($name) only accepts one argument, but calling greet('John','Doe') will fail. To fix it, pass only the required arguments or update the function definition to accept variable arguments using ...$args.

This error occurs when you try to access a class property that does not exist. For example, $obj->name will fail if name is not defined. To fix it, check your class definition and add the missing property, or use __get() magic method to handle dynamic property access safely.

This happens when you mistakenly treat an array as a callable function. For example, $arr = []; $arr(); will fail. To fix it, make sure you are calling a real function or callable. If you intended to access an array element, use $arr['key'] instead of function call syntax.