Memcache::add() stores variable
key only if such
key doesn't exist at the server yet.
Also you can use memcache_add() function.
The key that will be associated with the item.
The variable to store. Strings and integers are stored as is, other types are stored serialized.
Use MEMCACHE_COMPRESSED to store the item compressed (uses zlib).
Expiration time of the item. If it's equal to zero, the item will never expire. You can also use Unix timestamp or a number of seconds starting from current time, but in the latter case the number of seconds may not exceed 2592000 (30 days).
Returns TRUE on success or FALSE on failure. Returns FALSE if such key already exist. For the rest Memcache::add() behaves similarly to Memcache::set().