(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_key
     The 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.
key
     The key of the item to fetch.
cache_cb
     
       Read-through caching callback or NULL
      
cas_token
     The 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.