Manual:$wgDefaultUserOptions

The below changes to Localsettings.php will override the default user preferences for anonymous visitors and users who have not customized their preferences.

Available preferences
The following user preferences are available for customization:

Notes:
 * New settings are added to MediaWiki periodically. Some of these settings have been added since $wgDefaultUserOptions was introduced in v1.4.0.  Currently there is no documentation about when settings were added.
 * It is possible for extensions and language files to add extra toggles to the 'misc' tab. Languages that use this feature should have the setting described here.  Extensions should add the information to their extension page.
 * This list is not up to date with the latest MediaWiki Version. List options with --list.

Gadget options
gadget-$gname e.g. gadget-HotCat for MediaWiki:Gadget-HotCat.js

Default values
Each language sets its own default values for the user preferences in its language file. For example languages that read right-to-left will probably set the quickbar to appear on the right, instead of on the left. The values added to $wgDefaultUserOptions are overrides, i.e. if the value exists here then it will take precedence, but settings may be omitted from the array in which case the language's default value will be used.

In versions of MediaWiki prior to 1.8.0 the default value of this setting was simply an empty array, with the current language providing the default values.

From version 1.8.0 onwards, only the language-specific settings were overridden in the language file, so most settings are now explicitly set in.

From version 1.19, the overrides by the language files were removed because they didn't work, and the features that used them, like quickbar, have been done differently and better.

Example
To disable section editing links by default (for new and anonymous users), set the following in LocalSettings.php:

Add pages the user edits to their watchlist by default: