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 second line from the top (right below the  ) of :

Or set it in :

Or set in .htaccess:

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  ).

There are several configuration options to aid debugging. The following are all  by default. Enable them by setting them to  in your : MediaWiki will throw notices for some possible error conditions and for deprecated functions.
 * Enable more details (like a stack trace) to be shown on the "Fatal error" page.
 * Shows a toolbar on the page with profiling, log messages and more.
 * Adds the "log messages" part of wgDebugToolbar as a raw list to the page.

Turning display_startup_errors on
Some providers turn  off, which hides the errors even if you raise the   level. Turning it on within the program is too late! Instead you'll have to create a wrapper file around your file. In the case of MediaWiki you can just add this on top of mediawiki/index.php:

In other environments:

SQL errors
To log all SQL queries, rather than just the one that raised the exception, set  in :

Prior to MediaWiki 1.32, you needed to set  and  to see details of database exceptions in the HTML output:

Debugger
For the most common setup (using MediaWiki-Vagrant and PhpStorm) see .

Zend
If you are using the Zend PHP interpreter, you can debug your code with XDebug. MediaWiki-Vagrant has [<tvar|remote_debug_file>https://phabricator.wikimedia.org/diffusion/MWVA/browse/master/puppet/modules/php/manifests/remote_debug.pp;c377718dea960c5e9e7b3acc79893a5e3c14da01$27</> built in settings] for this. If you're not using MediaWiki-Vagrant, but your setup is similar, you can reuse those values. In some cases (e.g. due to a firewall), you may have to use the IDE on the same machine as the web server. In this case, you can simply set:

See the [<tvar|xdebug>http://xdebug.org/docs/remote</> XDebug documentation] for more information.

To debug a command-line script (e.g. PHPUnit, or a maintenance script) on MediaWiki-Vagrant, use:

Adjust the script, parameters, and remote host (it should be the IP of the computer where your IP is, 10.0.2.2 should work for MediaWiki-Vagrant) as needed.

Logging
For much greater detail, you need to profile and log errors.

Setting up a debug log file
To save errors and debugging information to a log, add <tvar|logfile> </> to the  file. Change the value to a text file where you want to save the debug trace output.

The MediaWiki software must have permissions from your operating system to create and write to this file, for example in a default Ubuntu install it runs as user & group :. Here's a sample setting:

This file will contain much debug information from MediaWiki core and extensions. Some subsystems write to custom logs, see 1>#Creating a custom log file</>|#Creating a custom log file to capture their output.

dbtrans>Special:MyLanguage/Database transactions</>|Database transaction lifecycle debugging can be enabled for some databases with <tvar|debugdb></>.

Creating a custom log file
Prior to MediaWiki 1.32, to create a custom log file that only holds your specific debug statements, use the  function. This function takes two arguments, the text string to log and the path to the log file:

Creating custom log groups
If you're debugging several different components, it may be useful to direct certain log groups to write to a separate file.

See <tvar|1></> for more information.

To set up custom log groups, use the following to LocalSettings.php:

To log to one of these groups, call  like this:

Structured logging
Structured logging allows you to include fields in your log records.

See <tvar|1></> for more information.

Statistics
Advanced client-side logging can be performed with <tvar|1></>, which requires a complex setup and careful inspection of privacy issues.

Simple counting of certain kind of events is possible (since MediaWiki 1.25) using StatsD. StatsD offers meters, gauges, counters, and timing metrics.

Usage example:

The metrics can be sent to a StatsD server, which may be specified via the  configuration variable. (If not set, the metrics are discarded.) You can work with StatsD locally (without needing a Graphite server) by starting a <tvar|statsd>StatsD</> server and configuring it with the "backends/console" backend, which will output metrics to the console.

As of MediaWiki 1.25, <tvar|incrStats> </> is a shortcut for the <tvar|inc> </> method on the main  instance.

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.



Working live with MediaWiki objects
<tvar|eval></> is an interactive script to evaluate and interact with MediaWiki objects and functions in a fully initialized environment.

$ php maintenance/eval.php > print wfMessage("Recentchanges")->plain; Recent changes

The <tvar|vagrant></> portable virtual machine integrates the interactive PHP shell  (when using Zend).

Callable updates
Code embedded in the  function, such as   in , is not executed during the web request, so no error message will be displayed if it fails.

For debugging, it may be helpful to temporarily remove the code from within the function so that it is executed live.

Client side debugging (JavaScript)
Wikipedia offers a rich set of tools for debugging client side JavaScript.

In addition to the MediaWiki tools, other techniques are available to assist with diagnosing client interactions.

Tools:


 * ResourceLoader offers 1>Special:MyLanguage/ResourceLoader/Developing with ResourceLoader#Debugging</>|a means to ensure JavaScript is easily viewable by client-side tools.


 * Open your browser's console. Many client side mediawiki scripts log error messages to the console using ResourceLoader, which provides 1>Special:MyLanguage/ResourceLoader/Modules#mediaWiki.log</>|a safety oriented way to log to the client console. Beyond the native JavaScript logging function, it provides a check to ensure that a console is available and that logging does not produce its own error.  1>Special:MyLanguage/ResourceLoader/Architecture#Debug_mode</>|ResourceLoader/Architecture#Debug_mode also describes this feature.


 * Browser tools may provide native functionality to debug client side script.


 * Network tracers, like [<tvar|url>https://www.wireshark.org/</> Wireshark] can provide insight into the script that is being provided by a page.