Manual:Lock the database

Since MediaWiki 1.5 you can add  to your LocalSettings.php somewhere after the line require_once("$IP/includes/DefaultSettings.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 does not prevent all writes to the database; if you need to prevent all writes, 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).