Manual:$wgSessionsInMemcached

Memcached settings: $wgSessionsInMemcached
Store session data in the object cache, rather than on the filesystem.
Introduced in version:pre 1.1.0
Deprecated in version:1.20.0 (Gerrit change 17929; git #6eb6313b)
Removed in version:1.29.0 (Gerrit change 324353; git #d6bdb6bb)
Allowed values:(boolean)
Default value:false

Details edit

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

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://php.net/function.session-set-save-handler