Manual:How to debug

This page gives a basic introduction to debugging MediaWiki software.

One of the first things you will notice is that "echo" generally does not work; this is part of the general design.

PHP errors
To see PHP errors, add this to the very top of LocalSettings.php:

This will cause PHP errors to be shown on-page. This might make it easier for attackers to find a way into your server, so disable it again when you have found the problem.

Note that fatal PHP errors may happen before the lines above are ever executed, or may prevent them from being shown. Fatal PHP errors are usually logged to Apache's error log – check the  setting in   (or use  ).

Any of the  type settings should be added to the bottom of your.

You can enable more details (like a stack trace) to be shown for some types of errors:

This is especially useful when debugging errors in the PHP code (as opposed to configuration problems).

Note that  does not include , so if you want to receive notices, you may need to use instead:

You may want to set, in :

Turning display_startup_errors on
display_startup_errors is off by default on the toolserver. Turning it on within the program under test is too late! So create the following stub and execute that instead.

SQL errors
To display SQL errors in error messages instead of "(SQL query hidden)", add the following to :

You can also enable backtrace on SQL error by setting $wgShowDBErrorBacktrace:

Write debug data to a debug log file
For much greater detail, you need to profile and log errors.

To save SQL errors to a log, add  to the   file. Change the value to a text file where you want to save the debug trace output.

This file will contain all of the built-in MediaWiki debug information as well as anything you try to log. To create a custom log file that only holds your debug statements, add this to LocalSettings.php.

Then debug to this custom log using a statement like this:

Send debug data to an HTML comment in the output
This may occasionally be useful when supporting a non-technical end-user. It's more secure than exposing the debug log file to the web, since the output only contains private data for the current user. But it's not ideal for development use since data is lost on fatal errors and redirects. Use on production sites is not recommended. Debug comments reveal information in page views which could potentially expose security risks.


 * Manual:$wgDebugComments

Profiling
To get more detail, you need to enable profiling. Profiling tracks code execution during a page action and reports back the percentage of total code execution that was spent in any specific function. The generated profile only includes functions that have specifically been marked to be profiled.

If you are not using profiling, but have a  file in the MediaWiki root folder, you may receive errors referring to. Deleting, or renaming, the  file will resolve this error. The  file, also in the MediaWiki root folder, can serve as a template should you enable profiling in the future.

To enable profiling, you need to modify the  (see   in the MediaWiki root folder for an example). By default the file includes a  which just dumps profiling information. To instead direct this information to a file, edit StartProfiler.php so that it looks like this:

Then you can customize profiling options  (not  ; be sure to edit beneath the requirement of  ).

Common configuration (both <1.7 and >1.8):

In MediaWiki 1.7 and earlier, instead of editing, you have to set   to. This will generate basic page timing information in the file defined by.

In addition to the settings list above, these additional settings are available:

Advanced profiling
Once you have enabled profiling, you can trace code execution through any function that you want to investigate as a bottleneck by wrapping the function with the following code:

After you've added this information, browse to a page in the wiki. This will generate profiling info in the log file you defined above. Change  in   to true or false for different display formats.

Logging to Database
To log profiling information to a database, first you'll have to create a profiling table in your MediaWiki database the table definition in the file maintenance/archives/patch-profiling.sql (the recommended way to do this is ). Then set  in LocalSettings.php.

Viewing Profile Info
If you log your profiling information to the database, you can view the information in a webpage by browsing to. You must also set in. Then, after gathering data by browsing wiki pages, visit  to see how much time your profiled code is using and how many times it's being called.

To view profiling information as HTML comments appended to the bottom of a page, just add  to the URL. This feature is not in the standard product, you can enable it by adding this to :

Useful debugging functions

 * – dies with an optional message and prints the call-stack

Debugging certain functionality

 * Resource Loader: ResourceLoader/Developing with ResourceLoader