Memcached::get() returns the item that was previously
stored under the key. If the item is found and
the get_flags is given Memcached::GET_EXTENDED,
it will also return the CAS token value for the item. See
Memcached::cas() for how to use CAS tokens.
Read-through caching callback may be
specified via cache_cb parameter.
The &cas_token parameter was removed.
Instead get_flags was added and when it is given the value of Memcached::GET_EXTENDED it will ensure the CAS token to be fetched.