SELECT *, max(post_date) FROM posts
         WHERE user_id IN ($matches)
         GROUP BY user_id
         ORDER BY post_date DESC

SELECT p.*
FROM posts p
JOIN(
SELECT user_id , max(post_date) post_date
FROM posts
GROUP BY user_id
) pp
USING(user_id,post_date)
WHERE pp.user_id IN ($matches) /* i assume here you have user ids like IN (1,2,3)*/
ORDER BY p.post_date DESC

"SELECT *, max(post_date) FROM posts
           WHERE user_id IN (".implode(',',$matches).")
           GROUP BY user_id
           ORDER BY post_date DESC "