Skip to main content

Exception Tracing

Since 4.4

Exceptions, and errors are a fact of life as a developer, and dealing with them in development should be as easy and painless as possible.

From Moodle 4.4 onwards Moodle can use the whoops library to display most Exceptions to developers.

The library is included with Composer, in the same way that other developer-only features are included, and is only enabled when debugging modes are enabled. It can also be disabled as a developer preference.

The whoops UI is only used when the following conditions are met:

  • $CFG->debugdisplay is true;
  • the script is not:
    • a CLI script using the CLI_SCRIPT constant
    • an AJAX script using the AJAX_SCRIPT constant
  • the error is not accessed:
    • during a PHPUnit test; or
    • during a Behat run.
  • the $CFG->debug_developer_use_pretty_exceptions value is not false
  • the whoops library is available (using composer install)

Debugging messages

Moodle typically show debugging messages created with the debugging() method inline, however this can be easy to miss.

If Whoops is configured then debugging() messages will instead trigger an error and be shown using the Whoops UI.

This can be controlled using the $CFG->debug_developer_debugging_as_error setting.


The use of whoops, and some features of it, are configurable to suit your personal preferences.

Disabling the UI

If you do not wish to use the whoops interface you can disable it by setting the following:

// Do not use Pretty Exception in the UI.
$CFG->debug_developer_use_pretty_exceptions = false;

The whoops UI can be configured to allow you to easily open files in your preferred editor using the "Open file" link in the UI. This can be configured in Moodle using the $CFG->debug_developer_editor property.

The following editors are available as standard:

  • emacs
  • idea
  • macvim
  • phpstorm
  • sublime
  • textmate
  • xdebug
  • vscode
  • atom
  • espresso
  • netbeans

For example:

$CFG->debug_developer_editor = 'vscode';
Adding your own editor

If your editor is not included in this list, but does support opening files using a URI handler then you can specify a callable which returns the URI, for example:

$CFG->debug_developer_editor = fn ($file, $line) => "whatever://open?file=$file&line=$line";

For full documentation on this feature, see the whoops documentation.

Treating debugging as an error

In normal circumstances Moodle will treat all calls to debugging() as an informational message which is shown inline in the page body.

You may wish to treat these as Errors instead, especially when updating code between Moodle versions.

The default behaviour for debugging() is:

  • if the whoops UI is installed, and available, then treat debugging() as an Error;
  • otherwise treat debugging() as an informational message only.

Thi scan be further configured by setting the $CFG->debug_developer_debugging_as_error property:

  • $CFG->debug_developer_debugging_as_error = true; - always treat debugging calls as an error, whether whoops is available or not;
  • $CFG->debug_developer_debugging_as_error = false; - never treat debugging calls as an Error, even when whoops is available.