( ! ) Notice: Undefined index: select in C:\...\facturer.php on line 18

( ! ) Notice: Undefined index: select in C:\Program Files\EasyPHP-5.3.5.0\www\comptes\facturer.php on line 18

$usersolde = MySql::getConnection()->query("SELECT SUM(credit - debit) AS soldera FROM comptes WHERE id_user = '".intval($_POST['select'])."' ")->fetch();

<?php
include("api.php"); if (Auth::isAdmin() == false) die("Vous devez &ecirc;tre un Administrateur et connect&eacute; pour avoir acc&egrave;s &agrave; cette page.");
if (!empty($_GET['deconnection']) && $_GET['deconnection'] == '1') {session_destroy(); header("Location: index.php");}

// On affiche après clique sur le bouton "Afficher" les objet du client
$err = false;
$msg = "";
$disp = false;
$del = false;
$files = array();
if ($_SERVER['REQUEST_METHOD'] != 'GET'){
if (isset($_POST['button']) && $_POST['button'] == 'Afficher'){
$disp = true;
$data =      MySql::getConnection()->query("SELECT id, username FROM utilisateurs WHERE id = '".intval($_POST['select'])."'"                   )->fetch();
$files =     MySql::getConnection()->query("SELECT * FROM comptes WHERE id_user = '".intval($_POST['select'])."' ORDER BY date_jour DESC"      )->fetch();}}
$tab =       MySql::getConnection()->query("SELECT id, username FROM utilisateurs"                                                             )->fetch();

$usersolde = MySql::getConnection()->query("SELECT SUM(credit - debit) AS soldera FROM comptes WHERE id_user = '".intval($_POST['select'])."' ")->fetch();

?>