Manual:Lock the database

Since MediaWiki 1.5 you can add  to your LocalSettings.php and set this to a string describing the reason for read-only mode. (Usual wiki markup is allowed in this string). This will prevent any editing on the wiki. Example:

Alternatively, set  in LocalSettings.php to the name of a file, then put the reason for the lock inside the file. The contents of the file will be displayed to anyone trying to edit the wiki.

Note that the read-only mode is not guaranteed to be absolutely safe; if you need to guarantee that MediaWiki will never write to the data, you need to do this on the database itself (e.g. revoke UPDATE, INSERT, DELETE from the wiki user, or put MySQL into read-only mode).

If you have the siteadmin permission, you can also use the special page Special:Lockdb to lock the database and Special:Unlockdb to unlock a previously locked database. Note that this is deprecated, and the siteadmin permission is not granted to any group per default. See Manual:User rights for information on granting permissions to groups. Also note that this works by writing resp. deleting the file specified by $wgReadOnlyFile, so make sure the file permissions are set appropriately.