无法调用多维数组

Question

我正在尝试使用Ajax和多维会话数组创建一个购物车系统。该数组存储该id的产品ID和数量。我试图显示其中一个存储的id的数量，但不返回正确的值。

这是我的PHP代码

public function create()
    {
        session_start();
        $product_id = $_GET['data'];

        // Check if the cart has been created
        if(isset($_SESSION['cart']) AND $_SESSION['cart'] != NULL)
        {
            // One or more items are have already been placed in the cart
        }else{
            // The cart is empty
            $_SESSION['cart'] = array();
            $_SESSION['cart'][$product_id] = array('quantity' => 3);
        }

        // Return the quantity of a stored product id
        return $_SESSION['cart'][$product_id]['quantity'];
    }

如您所见，应返回数字3。相反，我得到值1。