Manual:$wgSessionsInMemcached

Details
If 'true', MemcachedSessions.php is loaded on startup:


 * This file gets included if $wgSessionsInMemcached is set in the config.
 * It redirects session handling functions to store their data in memcached
 * instead of the local filesystem. Depending on circumstances, it may also
 * be necessary to change the cookie settings to work across hostnames.
 * * See: http://www.php.net/manual/en/function.session-set-save-handler.php