Memcache
PHP Manual

Предопределенные константы

Memcache Constants
Name Description
MEMCACHE_COMPRESSED (integer) Used to turn on-the-fly data compression on with Memcache::set(), Memcache::add() и Memcache::replace().
MEMCACHE_HAVE_SESSION (integer) 1 if this Memcache session handler is available, 0 otherwise.

Memcache
PHP Manual