Memcached settings: $wgUseMemCached
Use memcache, using the servers set in $wgMemCachedServers .
Introduced in version:before 1.1.0
Removed in version:1.17.0
Allowed values:(boolean)
Default value:false

Use Memcached memory-based object store.

With $wgUseMemCached = true;, old versions of MediaWiki will use the servers specified in the $wgMemCachedServers array.

Replaced by $wgMainCacheType



More information in docs/memcached.txt (and Memcached )

(1.3, 1.4) Plenty of code rely on this setting to indicate whether they should use the BagOStuff class (from $wgMemc) for caching, meaning they can not use Turck MMCache or eAccelerator, because this setting is only set if memcache is used, not if $wgMemc is valid. 1.5 may fix this.
(1.5) In all 1.5.x versions, this setting was not declared in DefaultSettings.php, but was still being used internally. This was rectified in 1.6.0.