Shopping Cart dg Session - Proses Add Cart

<?php
    // file proses/kategori-proses.php
    require_once("../config.php");
    if (isset($_GET['id'])) {
        $ada = false;
        foreach ($_SESSION['cart'] as $key => $item) {
            if ($key==$_GET['id']) {
                $ada=true;
            }
        }
        if ($ada) { // produk sdh ada di keranjang
            $_SESSION['cart'][$_GET['id']] += 1;
        } else { // produk blm ada di keranjang
            $_SESSION['cart'][$_GET['id']] = 1;
        }
        header("Location: ../?p=produk"); exit;
    }