Manual:$wgDefaultUserOptions

Details
Settings in this array will override the default user preferences for anonymous visitors and newly created accounts. When a new account is created the default settings in use at the time are stored along with the account details, so any subsequent changes to the default values will not affect existing users.

Available preferences
The following user preferences are available for customisation:

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.

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

The following code gives the default values as set in MediaWiki 1.8.0 - they may have been different in earlier versions, and would certainly have differed between languages.

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