Php Update mysql

Php Update mysql

Hello i've actually a table to update throught php/html and have no clue
why I got the error "Error" (which is my mysql_query die) I've tried if
every variable in my code were send in my update.php and everything was
here. So the problem come from my update.php code.
Here is my updating code:
<?php
include('config.inc.php');
$idn=$_POST["idn"];
$nom=$_POST['nom'];
$prenom=$_POST["prenom"];
$payss=$_POST["pays"];
$req="UPDATE membres SET prenom = '$prenom' , "
." pays = '$payss' "
." nom = '$nom' , "
." where id = $idn ";
$res=mysql_query($req) or die ("error updating!");
?>
Thanks for your answers !