(PECL memcached >= 0.1.0)
Memcached::add — Add an item under a new key
   Memcached::add() is similar to
   Memcached::set(), but the operation fails if the
   key already exists 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 already exists.