not the master key. MEMCACHED_FAILURE will be returned if no key is set. In the case
of a key which is too long MEMCACHED_BAD_KEY_PROVIDED will be returned.
+If you set a value with the value being NULL then the prefix key is disabled.
+
=item MEMCACHED_CALLBACK_USER_DATA
This allows you to store a pointer to a specifc piece of data. This can be