You mean besides querying about a billion things?
Code:
$query = "SELECT topic_id FROM phpbb_topics WHERE forum_id = 27 OR forum_id = 12 OR forum_id = 40 OR forum_id = 11 OR forum_id = 12 OR forum_id = 40";
12 and 40 are repeated. Not likely your magic solution — or much at all, actually — but it seems weird.