Ho risolto così
Codice PHP:
$query = @mysql_query("SELECT * FROM post INNER JOIN users ON post.id_user = users.id_user ORDER BY post.id DESC") or die('Errore: ' . mysql_error());                        while($risultato = mysql_fetch_array($query)) {                        $query2 = mysql_query("SELECT * FROM like1 WHERE id_user = $id_user AND id_post = ". $risultato['id']."");                           if(mysql_num_rows($query2) ==1) {            $like="dislike";                          } else {                        $like="like";                          }