вопрос
Убрать p.flag из запроса:
$db->query( "SELECT c.post_id, c.is_register, c.text, c.autor, c.email, p.id, p.date as newsdate, p.title, p.category, p.comm_num, p.alt_name FROM " . PREFIX . "_comments as c, " . PREFIX . "_post as p WHERE p.id=c.post_id AND c.approve=1 ORDER BY c.date DESC LIMIT 0, " . $max_comm );