Manual:$wgSessionsInMemcached

Details
Despite the name of this setting, it will use whatever object cache you specify in  .

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 the object cache
 * instead of the local filesystem. Depending on circumstances, it may also
 * be necessary to change the cookie settings to work across hostnames.
 * * See: https://secure.php.net/function.session-set-save-handler