(PECL memcached >= 0.1.0)
Memcached::getByKey — Retrieve an item from a specific server
$server_key
   , string $key
   [, callable $cache_cb
   [, float &$cas_token
  ]] )
   Memcached::getByKey() is functionally equivalent to
   Memcached::get(), except that the free-form
   server_key can be used to map the
   key to a specific server.
  
server_keyThe key identifying the server to store the value on.
keyThe key of the item to fetch.
cache_cb
       Read-through caching callback or NULL
      
cas_tokenThe variable to store the CAS token in.
   Returns the value stored in the cache or FALSE otherwise.
   The Memcached::getResultCode() will return
   Memcached::RES_NOTFOUND if the key does not exist.