$mpid = (int) $_GET['mpid']; //On récupère la valeur de l'id
    //La requête nous permet d'obtenir les infos sur ce message :
    $query = $cnx->prepare('SELECT users_id_sender, users_id_receiver, mp_id, mp_title, mp_post, mp_date, mp_read, response_mp_id
    FROM t_mp
    WHERE mp_id = :id');
    $query->bindValue(':id',$mpid,PDO::PARAM_INT);
    $query->execute();
	// si response_mp_id est définit, c'est une réponse
	if (isset($data['response_mp_id'])) {
		$idmp=$data['response_mp_id'];
	}// sinon c'est le message prère
	if (!isset($data['response_mp_id'])) {
		$idmp=$mpid;
	}

....

    $query = $cnx->prepare('SELECT users_id_sender, users_id_receiver, mp_id, mp_title, mp_post, mp_date, mp_read, response_mp_id
    FROM t_mp
    WHERE mp_id = :idmp || response_mp_id = :idmp');
    $query->bindValue(':idmp',$idmp,PDO::PARAM_INT);

    $query = $cnx->prepare('SELECT users_id_sender, users_id_receiver, mp_id, mp_title, mp_post, mp_date, mp_read
    FROM t_mp
    WHERE mp_title = :titlemp');
    $query->bindValue(':titlemp',$titlemp,PDO::PARAM_STR);
    $query->execute();
	while ($data = $query->fetch())
	{
        }

CREATE TABLE IF NOT EXISTS `t_mp` (
  `mp_id` int(11) NOT NULL AUTO_INCREMENT,
  `users_id_receiver` int(11) NOT NULL,
  `users_id_sender` int(11) NOT NULL,
  `mp_title` varchar(45) NOT NULL,
  `mp_post` longtext NOT NULL,
  `mp_date` datetime NOT NULL,
  `mp_read` enum('0','1') NOT NULL,
  PRIMARY KEY (`mp_id`),
  KEY `t_mp_FKIndex1` (`users_id_receiver`),
  KEY `t_mp_FKIndex2` (`users_id_sender`),
  KEY `fk_t_users3` (`users_id_receiver`),
  KEY `fk_t_users4` (`users_id_sender`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=3 ;

$data = $query->fetch();
	{
	if (preg_match('#^(Re: )*$#',$data['mp_title'])) {$mptitle='Re: '.$data['mp_title'];}
	if (!preg_match('#^(Re: )*$#',$data['mp_title'])) {$mptitle=$data['mp_title'];}
...
...
       }