(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 or retrieve it from. Instead of hashing on the actual key for the item, we hash on the server key when deciding which memcached server to talk to. This allows related items to be grouped together on a single server for efficiency with multi operations.
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.