La récupération PDO ne renvoie rien

Selon la documentation

Dans votre cas, vous devriez utiliser quelque chose comme

$query = $db->prepare('(SELECT last_visit, 
                               last_ip 
                        FROM user_log 
                        WHERE user_id = :id_1
                       ) 
                        UNION
                       (SELECT time AS last_visit,
                               packet_hex AS last_ip 
                        FROM crack_log 
                        WHERE target_id = :id_2
                       )
                       ORDER BY last_visit DESC LIMIT 0,10;'
                     );
    $query->execute(array(':id_1'=> $_SESSION['id'],
                          ':id_2'=> $_SESSION['id'] 
                         )
                   );