Manual:$wgReadOnly

Details
Functionally this setting is the same as  , but uses a variable instead of a file.

If set to a string in "LocalSettings.php",  disallows editing, displaying the string given as the reason. The reason provided is preceded by the contents of MediaWiki:Readonlywarning.

The above example locks the database and provides a readonlyWarning>MediaWiki:Readonlywarning|default statement on edit pages and also gives the included string  as an explanation for why the database was locked.

In contrast to its name, this settings does not make the database read only! Even if $wgReadOnly is set, extensions, API scripts and other cachable events can write data nonetheless.

DB caching
Some caches that use the database may still be written to during read only mode. This can be avoided by disabling cache, albeit at the cost of making your wiki slow. This can be accomplished with the following settings in LocalSettings.php:

If you want your wiki to be fast, while still not using the DB, you should consider using something like Memcached. See   and   for details.