Memcache::replace() should be used to replace value of
existing item with key. In case if item with such
key doesn't exists, Memcache::replace() returns
FALSE. For the rest Memcache::replace() behaves
similarly to Memcache::set().
Also you can use memcache_replace() 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.
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).