(PECL memcached >= 2.0.0)
Memcached::incrementByKey — Increment numeric item's value, stored on a specific server
$server_key
   , string $key
   [, int $offset = 1
   [, int $initial_value = 0
   [, int $expiry = 0
  ]]] )
    Memcached::incrementByKey() increments a numeric item's value
   by the specified offset. If the item's value is not
   numeric, an error will result.
    Memcached::incrementByKey() will set the item to the
   initial_value parameter if the key doesn't exist.
  
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 increment.
offset
     The amount by which to increment the item's value.
initial_value
     The value to set the item to if it doesn't currently exist.
expiry
     The expiry time to set on the item.
   Returns new item's value on success or FALSE on failure.