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<?php
session_start(); 
$n = count($_SESSION['panier']); 
if($n != 0) {
$n++; 
}
if ((isset($_POST['id']))and (isset($_POST['quantite'])))
{
$_SESSION['panier'][$n]['id']=$_POST['id']; 
$_SESSION['panier'][$n]['quantite'] = $_POST['quantite'];
}

$n = count($_SESSION['panier']);
for ($i = 0; $i<$n; $i++) {
?>
<br/>
<?php
echo $_SESSION['panier'][$i]['id'];
?>
<br/>
<?php
echo $_SESSION['panier'][$i]['quantite'];
?>
<br/>
<?php

}
?>

echo $_SESSION['panier'][$i]['id'];
echo $_SESSION['panier'][$i]['quantite'];

$n = count($_SESSION['panier']); // = 0
if($n != 0) { // non exécuté
   $n++; 
}

if ((isset($_POST['id']))and (isset($_POST['quantite']))) {
  // == $_SESSION['panier'][0]['id']
  $_SESSION['panier'][$n]['id']=$_POST['id']; 
  
  // == $_SESSION['panier'][0]['quantite']
  $_SESSION['panier'][$n]['quantite'] = $_POST['quantite']; 
}
// == 1 article dans le panier

$n = count($_SESSION['panier']); // == 1
if($n != 0) { // exécuté
   $n++; // $n = 2
}

if ((isset($_POST['id']))and (isset($_POST['quantite']))) {
  // == $_SESSION['panier'][2]['id']
  $_SESSION['panier'][$n]['id']=$_POST['id']; 
  
  // == $_SESSION['panier'][2]['quantite']
  $_SESSION['panier'][$n]['quantite'] = $_POST['quantite']; 
}
// == 2 articles dans le panier

for ($i = 0; $i<$n; $i++) { // == for ($i = 0; $i<2; $i++)
  // l'index 1 n'existe pas !! (index existants : 0 et 2)
}