Manual:Lock the database

Since MediaWiki 1.5 you can add  to your  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 the "LocalSettings.php" file 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).