Update class.Cart.php

[nia-cloud-official]

This update introduces several enhancements to the cart system, focusing on improving the user experience and adding functionality for discount codes, shipping costs, and persistent carts.

1. Discount Feature

   Added functionality to apply a discount code to the cart, reducing the total price.
   Users can input a discount code, and the cart will automatically apply the discount to the total.
   New methods:
   applyDiscount($code, $amount): Applies the discount code and amount.
   getTotalWithDiscount(): Returns the total price after applying the discount.

2. Shipping Costs

   Implemented a way to add shipping costs to the cart total.
   The shipping cost can be set manually, and it will be added to the total amount at checkout.
   New methods:
   setShippingCost($cost): Sets the shipping cost.
   getTotalWithShipping(): Returns the total cost after adding shipping.

3. Persistent Cart for Logged-in Users

   Added the ability to save the cart data into a database for logged-in users.
   This allows users to maintain their cart items between sessions, ensuring a smoother shopping experience.
   New methods:
   saveToDatabase($userId): Saves the cart items to the database for the given user.
   loadFromDatabase($userId): Loads the cart from the database when the user logs in.

Bug Fixes

Refined the session writing and reading mechanism to ensure data integrity across different cart updates.
Improved the getTotalQuantity() and getTotalItem() methods to be more accurate when applying discounts and shipping costs.