(PECL memcached >= 2.0.0)
Memcached::decrementByKey — Decrement numeric item's value, stored on a specific server
$server_key,$key,$offset = 1,$initial_value = 0,$expiry = 0
Memcached::decrementByKey() decrements a numeric item's
value by the specified offset. If the item's value
is not numeric, an error will result. If the operation
would decrease the value below 0, the new value will be 0.
Memcached::decrementByKey() will set the item to the
initial_value parameter if the key doesn't exist.
server_key本键名用于识别储存和读取值的服务器。没有将实际的键名散列到具体的项目，而是在决定与哪一个 memcached 服务器通信时将其散列为服务器键名。这使得关联的项目在单一的服务上被组合起来以提高多重操作的效率。
keyThe key of the item to decrement.
offsetThe amount by which to decrement the item's value.
initial_valueThe value to set the item to if it doesn't currently exist.
expiryThe expiry time to set on the item.
Returns item's new value on success 或者在失败时返回 false.