Debugging PHP Code - GeeksforGeeks

Debugging PHP Code

Last Updated : 2 Nov, 2026

Debugging PHP scripts involves identifying and fixing errors that prevent a program from working as expected. PHP developers can use error reporting, logging, debugging functions, and tools such as Xdebug to find problems and inspect program execution.

  • Syntax errors: Check for missing semicolons, unmatched brackets, and invalid PHP syntax.
  • Logical errors: Verify conditions, operators, variable values, and function behavior.
  • Database errors: Inspect SQL queries and database error messages when a query fails.
  • Runtime errors: Use error reporting and logs to identify problems that occur during execution.
  • Debugging tools: Use an IDE debugger to inspect variables, set breakpoints, and execute code step by step.

1. Check Common PHP Coding Errors

Many PHP problems result from small mistakes in syntax, variable names, or conditional expressions.

Missing Semicolons and Brackets

PHP statements generally end with a semicolon, and opening braces must have matching closing braces.

PHP 
<?php
$name = "Alen";
echo $name;
?>

Use the syntax highlighting and diagnostics provided by your code editor to identify missing semicolons, unmatched brackets, and other syntax problems.

Incorrect Variable Names

PHP variable names are case-sensitive. For example, $name and $Name refer to different variables.

PHP 
<?php
$name = "Alen";

// Different variable
echo $Name; 
?>

Check variable names carefully when a script produces unexpected values or warnings.

Incorrect Assignment and Comparison Operators

The = operator assigns a value, whereas == compares values.

PHP 
<?php
$a = 5;
$b = 5;

if ($a == $b) {
    echo "Values are equal";
}
?>

Using = instead of == in a condition assigns a value rather than comparing two values. Use === when both value and type must match.

2. Debug SQL Queries

Database errors can occur because of invalid SQL syntax, incorrect table names, or connection problems. When using MySQLi, check whether a query succeeds and inspect the error message if it fails.

PHP 
<?php
$conn = new mysqli("localhost", "root", "", "test_db");

if ($conn->connect_error) {
    error_log("Database connection failed: " . $conn->connect_error);
    exit("Unable to connect to the database.");
}

$sql = "SELECT * FROM users";

if (!$conn->query($sql)) {
    error_log("SQL query failed: " . $conn->error);
} else {
    echo "Query executed successfully.";
}

$conn->close();
?>
  • Check the database connection before executing queries.
  • Use $conn->error to inspect a failed MySQLi query.
  • Log detailed database errors for debugging rather than displaying them to end users.
  • Use prepared statements when queries include user-supplied values.

3. Check PHP Errors and Logs

PHP provides error reporting and logging settings to help identify problems during execution.

During development, you can enable error reporting in a PHP script:

PHP 
<?php
error_reporting(E_ALL);
ini_set('display_errors', '1');
ini_set('log_errors', '1');
?>
  • error_reporting(E_ALL): Reports all supported error levels.
  • display_errors: Displays errors in the response.
  • log_errors: Enables error logging.

For production environments, set display_errors to Off and keep log_errors enabled. Check the configured PHP error log to investigate issues without exposing internal details to visitors.

4. Debug Variables Using PHP Functions

PHP provides built-in functions to inspect variable values and data structures.

Using var_dump()

The var_dump() function displays a variable's type and value.

PHP 
<?php
$user = [
    "name" => "Alen",
    "age" => 25
];

var_dump($user);
?>

Using print_r()

The print_r() function displays arrays and objects in a readable format.

PHP 
<?php
$user = [
    "name" => "Alen",
    "age" => 25
];

print_r($user);
?>

Use these functions during development to inspect data and identify unexpected values. Remove unnecessary debugging output before deploying the application.

5. Check PHP Syntax Using the Command Line

PHP provides a syntax-checking option that checks a file for syntax errors without executing it.

Run the following command in a terminal:

php -l index.php

Possible output when the syntax is valid:

No syntax errors detected in index.php

If the file contains a syntax error, PHP reports the error and its location. This is useful for detecting missing semicolons, unmatched brackets, and other syntax problems before running a script.

If the php command is not recognized, verify that PHP is installed and that the PHP executable is available through your system's PATH or configured in your editor.

6. Debug PHP Scripts Using Xdebug

Xdebug is a PHP extension that supports step debugging, allowing developers to pause execution, inspect variables, and follow the program's execution flow.

Set Up Xdebug in VS Code

1. Install the PHP Debug extension: Open the Extensions view in VS Code and install the PHP Debug extension.

2. Install Xdebug: Follow the official Xdebug installation guide to install a version compatible with your PHP installation.

3. Configure Xdebug: Add the following settings to the php.ini file used by your PHP environment.

zend_extension=xdebug
xdebug.mode=debug
xdebug.start_with_request=yes
xdebug.client_port=9003

The extension path may need to be specified explicitly, depending on your PHP installation. The port 9003 is Xdebug's default debugging port.

4. Configure VS Code: Open the Run and Debug view, select create a launch.json file, and choose PHP.

5. Add a debugging configuration: Ensure that the .vscode/launch.json file contains a configuration similar to the following.

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Listen for Xdebug",
            "type": "php",
            "request": "launch",
            "port": 9003
        }
    ]
}

6. Start debugging: Select Listen for Xdebug and start the debugger. Set a breakpoint in your PHP file and execute the script through your browser or another configured PHP execution environment.

Xdebug connects to the IDE, allowing you to pause execution at breakpoints and inspect variables. The PHP Debug extension provides the VS Code integration, while Xdebug provides the debugging functionality.

Common Xdebug Features

  • Breakpoints: Pause execution at a selected line of code.
  • Step Over: Execute the current line and move to the next line without stepping into function calls.
  • Step Into: Enter a function to inspect its execution.
  • Step Out: Continue execution until the current function returns.
  • Variable inspection: Examine local variables and other available values while execution is paused.

Best Practices for Debugging PHP Scripts

  • Use a code editor with syntax highlighting, bracket matching, and PHP diagnostics.
  • Check error messages and line numbers before changing code.
  • Use php -l to identify syntax errors quickly.
  • Use var_dump() or print_r() to inspect unexpected variable values during development.
  • Use Xdebug breakpoints when you need to examine execution flow or investigate complex bugs.
  • Log errors in production instead of displaying detailed error messages to visitors.
  • Remove temporary debugging statements before deploying your application.
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