Problème de session (PHP)

<?php
if (isset($_POST['connexion']))
{
   if ((isset($_POST['mdp2']) && !empty($_POST['mdp2'])) && (isset($_POST['mail2']) && !empty($_POST['mail2'])))
   {
      $base = mysql_connect ('localhost', '502533_admin', 'hyt9tcz*130108');
      mysql_select_db ('fleepy_clanteam_db', $base);

      $sql = 'SELECT id FROM membres WHERE mail="'.$_POST['mail2'].'"';
      $req = mysql_query($sql) or die(mysql_error());
      $data = mysql_fetch_array($req);

      if ($data[0] == 0)
      {
         $erreur = "Cette adresse électronique n'est liée à aucun compte !";
      }
      else
      {
         $sql = 'SELECT id FROM membres WHERE mail="'.$_POST['mail2'].'" AND mdp="'.sha1($_POST['mdp2']).'"';
         $req = mysql_query($sql) or die(mysql_error());
         $data = mysql_fetch_array($req);

         if(mysql_num_rows($req) > 0)
         {
            $erreur = "Le mot de passe est incorrect !";
         }
         else
         {
            $data = mysql_fetch_array($req);

            session_start();
            $_SESSION['id'] = $data['id'];
            $_SESSION['nom'] = $data['nom']; 
            $_SESSION['prenom'] = $data['prenom'];
            $_SESSION['date_naissance'] = $data['date_naissance'];
            $_SESSION['sexe'] = $data['sexe'];
            $_SESSION['mail'] = $data['mail'];
            $_SESSION['mdp'] = sha1($data['mdp']);
            $_SESSION['niveau'] = $data['niveau'];

            mysql_free_result($req); 
            mysql_close();

            header('Location: accueil.php'); 
            exit(); 
         }
      }
   }
   else
   {
      $erreur = "Vous devez remplir tout les champs !";
   }
}
?>