(PECL memcached >= 0.1.0)
Memcached::casByKey — Compare and swap an item on a specific server
$cas_token
   , string $server_key
   , string $key
   , mixed $value
   [, int $expiration
  ] )
    Memcached::casByKey() is functionally equivalent to
    Memcached::cas(), except that the free-form
   server_key can be used to map the
   key to a specific server. This is useful if you need
   to keep a bunch of related keys on a certain server.
  
cas_token
     Unique value associated with the existing item. Generated by memcache.
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 under which to store the value.
value
     The value to store.
expiration
     The expiration time, defaults to 0. See Expiration Times for more info.
   Returns TRUE on success or FALSE on failure.
   The  Memcached::getResultCode() will return
   Memcached::RES_DATA_EXISTS if the item you are trying
   to store has been modified since you last fetched it.