AND category='3' OR category='5' OR category='6' OR category='7' OR category='8' 

$db->query("SELECT id, title, date, alt_name, category, flag FROM " . PREFIX . "_post WHERE approve='1' 
AND date >= '$this_month' - INTERVAL 1 MONTH AND date < '$this_month' AND category='3' OR category='5' OR category='6' 
OR category='7' OR category='8' ORDER BY date DESC, comm_num DESC, rating DESC, news_read DESC LIMIT 0,10");