This method also returns false in case you set the value to false, so in order to have a proper fault mechanism in place you need to check the result code to be certain that a key really does not exist in memcached.
$Memcached = new Memcached();
var_dump($Memcached->get('key')); // boolean false
var_dump($Memcached->getResultCode()); // int 0 which is Memcached::RES_SUCCESS
Or just make sure the values are not false :)