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