cart.php

<?php
session_start();

$products = [];
$products[1] = array('name' => 'Bar Soap', 'price' => '4.50', 'weight' => '4');
$products[2] = array('name' => 'Lotion', 'price' => '9.99', 'weight' => '8');
$products[3] = array('name' => 'Liquid Soap', 'price' => '12.00', 'weight' => '10');

function product_exists($product_id) {
    global $products;
    return array_key_exists($product_id, $products);
}

function product_in_cart($product_id) {
    return array_key_exists($product_id, $_SESSION['cart']);
}

$product_id = $_GET['product_id']; //the product id from the URL
$action = $_GET['action']; //the action from the URL

switch ($action) {
    case 'add':
        if (product_exists($product_id)) {
            if (!isset($_SESSION['cart'][$product_id])){
                $_SESSION['cart'][$product_id] = 0;
            }
            $_SESSION['cart'][$product_id]++;
        }
        break;
    case 'update':
        $quantity_array = $_POST['quantity'];
        if (isset($quantity_array) && is_array($quantity_array)) {
            foreach($quantity_array as $product_id => $quantity) {
                if (product_exists($product_id) && product_in_cart($product_id)) {
                    $_SESSION['cart'][$product_id] = $quantity;
                }
            }
        }
        break;
    case 'remove':
        unset($_SESSION['cart'][$product_id]);
        break;
    case 'empty':
        unset($_SESSION['cart']);
        break;
}
?>