Vérifié si une valeur vos NULL

 function accepter_demande()
 {
   $count_user = 0;
	    $req = $bdd->prepare('SELECT active FROM amis WHERE pseudo_exp = :pseudo_exp AND pseudo_dest = :pseudo_dest OR pseudo_exp = :pseudo_exp_get AND pseudo_dest = :pseudo_dest_sess');
        $req->execute(array(
        'pseudo_dest' => $_GET['pseudo'],
        'pseudo_exp' => $_SESSION['pseudo'],
		'pseudo_exp_get' => $_GET['pseudo'],
		'pseudo_dest_sess' => $_SESSION['pseudo']));
    while($count_user = $req->fetch())
    {
    ++$count_user;
    }
    $req->closeCursor();
	}