(PECL memcached >= 0.1.0)
Memcached::replace — Replace the item under an existing key
Memcached::replace() is similar to
Memcached::set(), but the operation fails if the
key does not exist on the server.
keyThe key under which to store the value.
valueThe value to store.
expirationThe 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_NOTSTORED if the key does not exist.