[Sergei]

Код:

SELECT gid,uid FROM mybb_players WHERE username='%s' LIMIT 1

Код:

SELECT * FROM mybb_players WHERE uid=%d AND gid=(SELECT uid FROM mybb_users WHERE uid=%d AND password=MD5('%s')) LIMIT 1

And start naming fields with some sense like user_id, player_id, etc.

mysql_users
user_id

mysql_players
player_id
user_id