问更新购物车功能添加新项目，而不是更新现有项目

EN

public function AddToCart()
{
    if($_SERVER['REQUEST_METHOD'] == 'POST') {
        $quantity = 1;
        $prod_id = $_POST['prod_id'];
        $prod_data = $this->ProductFendModel->GetProductByIdCart($prod_id);

        $data = [
            'prod_name' => $prod_data->prod_name,
            'sale_price' => $prod_data->sale_price,
            'sp_name' => $prod_data->sp_name    
        ];

        $response = array();
        $message = '';    
        $key = "{$prod_id}";

        if (empty($_SESSION['cart_array'][$key])) {
            $_SESSION['cart_array'][$key] = array(
                'prod_id' => $prod_id,
                'quantity' => $quantity,
                'prod_name' => $data['prod_name'],
                'sale_price' => $data['sale_price'],
                'sp_name' => $data['sp_name']
            );
        }
        else {
            $_SESSION['cart_array'][$key]['quantity'] += $quantity;
        }

        $response['success'] = true;
        $response['message'] = 'Product added to cart';

        echo json_encode($response);
    }
}

public function UpdateCart()
{
    if($_SERVER['REQUEST_METHOD'] == 'POST') {
        foreach($_POST['prod_id'] as $key => $id)
        {
            $qty = $_POST['quantity'][$key];
            $_SESSION['cart_array'][$key]['quantity'] = $qty;
        }
    }
}