Uniquely identifying an array... as a key to another array

Alright, here's the situation.  I'm wrapping memcached, which takes a list
of several servers, into a quasi-singleton.  In other words, I want only one
instance per set of servers, not per server.

Assuming I had three memcached servers at localhost, 192.168.1.1 and
192.168.1.2, I wouldn't want one instance per server, but given that there
are seven possible permutations of combinations / only one server
configuration, MemcacheConnector::$instances could be an array with seven
objects.  I'm expecting MemcacheConnector::getInstance($array) to take an
array formatted like 'server' => 'port'.

Thusly, the question then becomes, what's the lightest possible way to store
a unique-ish array as the key for MemcacheConnector::$instances?  Having
MemcacheConnector::getInstance() check for
self::$instances[serialize($serverArray)] seems a huge waste of both ram and
CPU time.  Is there a better way?


Thanks,
Eddie