PHP Dev Tools (for CodinGame or elsewhere)


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Finding bugs (Xdebug)

Xdebug logo


If you start any puzzle in CodinGame, the starter code contains the following line, suggesting how to debug:

// To debug (equivalent to var_dump): error_log(var_export($var, true));

Printing out messages or variable values to the error log is fine for a simple puzzle script. But sometimes we have a non-trivial bug and a bit more complex code or data structure, when such debug output becomes hard to manage.

As we have already installed php and VS Code locally, by adding Xdebug we will be able to:

  • Set breakpoints (even conditional ones) in our code using the IDE.
  • When we run our code locally, php will pause when reaching a breakpoint. The IDE will show the source code line where the execution paused.
  • We can inspect the current values of variables and objects.
  • We can continue running our code till another breakpoint is reached.
  • ...or we can run our code step-by-step, one source code at a time (either jumping into function calls, or stepping over them).
  • When we found the bug, just exit the debug mode, correct the source code and re-run to check if the problem is solved or not.

Installing Xdebug

Again, I will not go into full details. The necessary steps are depending on OS, but the Xdebug website has detailed documentation. The following steps are valid for a Windows installation:

  • Download the latest Windows, 64-bit binary from the download page.
    • Match with the major version of your local php installation, e.g. 8.1.
    • For XAMPP, use the thread-safe (TS) build.
  • Rename this file to php_xdebug.dll. (So that we won't need to change the reference in the config file after every minor version update.)
  • Move the file to the /ext/ subdirectory of your local php installation (in my case to c:\xampp\php\ext\).
  • Add these settings to your php.ini file:
xdebug.start_with_request = yes
xdebug.client_port = 9003
xdebug.client_host = ""
xdebug.log = "C:\xampp\tmp\xdebug.log" ; or your tmp directory
xdebug.idekey = VSCODE

In a previous chapter I recommended to enable JIT. Unfortunately, JIT and Xdebug does not work well together, so I suggest to use TWO separate php.ini files:

  • The default php.ini is used when we don't want to debug, just to run the code (as fast as possible):
; zend_extension=xdebug
  • On the other hand, php-debug.ini has opcache commented out, but xdebug enabled. Otherwise the two ini files are identical.
; zend_extension=opcache
  • I also have a phpd.bat batch file in a directory that is in my PATH, together with the php-debug.ini file:
@echo off
php -c %~dp0/php-debug.ini %*

So when I need to debug, I can just type phpd instead of php.

Beware: with xdebug enabled, your code runs 5x to 10x slower. So use debug mode ONLY when you are really debugging.

Debugging in VS Code

  • Add the PHP Debug extension to VS Code. (See the previous chapter of this playground)
  • Choose Run / Add configuration... menu, select "Listen for Xdebug"
    • The port should match what you have set for xdebug.client_port in the php.ini file, e.g. 9003
  • Set a breakpoint by clicking on the left side of a source line.
    • A red circle should appear next to the line number.
    • Beware: if this line is in the middle of the loop, your run will pause at EVERY iteration. If that was not what you wanted, you can set up a conditional breakpoint, e.g. pause only if the loop variable equals a specific value.
  • Select Run / Start debugging menu, choose the "Listen for Xdebug" config created earlier.
    • The status line at the bottom of the VS Code window changes its color, showing it is now listening to a debug event from Xdebug.
  • Open a Terminal window (either within VS Code or separately).
  • Run your code from the command line:
phpd my_solution.php < input_01.txt > output_01.txt 2>&1
  • Note, that php is replaced with phpd, the batch file we created above, so it will use the php config file with Xdebug enabled.
  • If everything is OK, the execution should pause at the breakpoint, and VS Code shows the source code line with the breakpoint highlighted.
  • Now you can use all the debug facilities: step over, inspect variable, etc. Find a VS Code tutorial if needed. This part of VS Code usage is not specific to php.

Note: besides the 'Step Debugging' described above, Xdebug has additional features, such as Code Coverage Analysis, Garbage Collection Analysis, Profiling, Function Trace. These can be turned on with the xdebug.mode setting in the ini file, but are beyond our scope, so check the Xdebug documentation for details on how to use them.

Useful links

Coming next

"De gustibus non est disputandum." (There is no accounting for taste.) But did the Romans think about coding styles?

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