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).
Return Values
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().