ShoppingCart - provides a global way to interface with the cart (current order).

This can be used in other code by calling $cart = ShoppingCart::singleton();

The shopping cart can be accessed as an order handler from the back-end
(e.g. when creating an order programmatically), while the accompagnying controller
is used by web-users to manipulate their order.

A bunch of core functions are also stored in the order itself.
Methods and variables are in the shopping cart if they are relevant
only before (and while) the order is placed (e.g. latest update message),
and others are in the order because they are relevant even after the
order has been submitted (e.g. Total Cost).

Key methods:

//get Cart
$myCart = ShoppingCart::singleton();

//get order
$myOrder = ShoppingCart::current_order();

//view order (from another controller)

//add item to cart