Manual:$wgDefaultUserOptions

Details
Settings added to this array will override the language globals for the user preferences used by anonymous visitors and newly created accounts. These may vary from language to language, but the default setting for english are as follows:

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.

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