Memcache
PHP Manual

Predefined Constants

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

Memcache
PHP Manual