Debugging PHP Code: Techniques and Tools for Pros

Introduction: Why Debugging is a Developer's Superpower

Every PHP developer has encountered the white screen of death, an inexplicable 500 Internal Server Error, or a variable that "was just working a moment ago" at least once. In moments like these, you want to throw your laptop out the window. But instead, let's turn PHP code debugging into a routine that doesn't make your knees shake.

In this article, we'll break down techniques and tools that will help you find bugs faster than a kettle boils. From the classic var_dump() to advanced Xdebug configuration — all with live examples and real-world cases. Let's go!

1. Basic Debugging: From var_dump() to error_reporting

1.1. var_dump() and print_r() — Old-School but Effective Methods

The simplest way to debug PHP code is to output the contents of a variable to the screen. Yes, it's not elegant, but when you need to quickly understand what's in an array, var_dump() does its job 100%.

<?php\n$users = [\n    ['name' => 'Ivan', 'age' => 25],\n    ['name' => 'Maria', 'age' => 30]\n];\n\n// var_dump shows data types and structure\nvar_dump($users);\n\n// print_r — a more compact option\necho '<pre>';\nprint_r($users);\necho '</pre>';\n?>

Tip: Use <pre> around the output to preserve formatting. And never forget to remove such debug lines before deploying to production — otherwise users will see your "skeletons in the closet."

1.2. Enable Display of All Errors

The standard PHP configuration often hides errors "under the covers." To see them, add this to the beginning of your script:

<?php\n// Show all errors, including notices and warnings\nerror_reporting(E_ALL);\nini_set('display_errors', 1);\n?>

This is especially useful when debugging PHP code on a local server. But on production, be sure to disable display_errors — it's a security risk!

1.3. debug_backtrace() — Who Called This Function?

Sometimes a function is called from the wrong place. Stack trace helps find the source:

<?php\nfunction logError($message) {\n    $trace = debug_backtrace();\n    echo "Error in file: {$trace[0]['file']} on line {$trace[0]['line']}<br>";\n    echo "Message: $message";\n}\n\n// Now any call to logError will show where it came from\nlogError('Something went wrong');\n?>

2. Xdebug — The Swiss Army Knife for PHP Debugging

2.1. Installing and Configuring Xdebug

Xdebug is a PHP extension that turns debugging into a spaceship. With it, you can set breakpoints, view variable values in real-time, and even profile code.

Installation via pecl (for Linux/Mac):

pecl install xdebug

For Windows, it's easier to download the .dll from the official website and add it to php.ini:

zend_extension = /path/to/xdebug.so\nxdebug.mode = debug\nxdebug.start_with_request = yes

2.2. Breakpoints in IDE

If you use PhpStorm or VS Code, configure the integration with Xdebug. Set a breakpoint (red circle to the left of the code line), start debugging — and the code stops at that line. You see all variables, can step through, and even change values "on the fly."

Example: You're looking for why the order total is calculated incorrectly. Set a breakpoint on the calculation line, run the request — and you see that $discount is null instead of a number. Problem found in 10 seconds!

2.3. Profiling with Xdebug

Slow code? Enable the profiler:

xdebug.mode = profile\nxdebug.output_dir = /tmp/profiles

After running the script, you'll get