+ :param ptr: pointer to an initialized `memcached_st` struct
+ :param group_key: key namespace
+ :param group_key_length: length of the key namespace without any terminating zero
+ :param key: the key
+ :param key_length: length of the key without any terminating zero
+ :param value: the value to append/prepend
+ :param value_length: the length of the value without any terminating zero
+ :param expiration: expiration as a unix timestamp or as relative expiration time in seconds
+ :param flags: 16 bit flags
+ :returns: `memcached_return_t` indicating success
+