Web Database Applications with PHP and MySQL, 2nd Edition

Finalizing Orders

After entering valid credit card details and optional delivery instructions, the next step in the ordering process is to turn the shopping cart into an order that's owned by a customer. This requires several steps that include checking that sufficient inventory is available to complete the order and deducting the wines sold from the inventory.

The order finalization requires locking of the database and is an example of the moderately complex query processing that described in Chapter 8. Locking prevents several undesirable problems. For example, locking prevents the same inventory being purchased by two users at the same time. It also prevents unlikely problems such as the same user finalizing two separate orders at the same time in two browser windows and confusing the application.

The script order/order-step3.php shown in Example 19-3, later in this section, performs the ordering process. The script works as follows:

1. It tests that the user is logged in (using the sessionAuthenticate( ) function discussed in Chapter 20) and that the cart has contents. These tests should never fail, as the Make Purchase button is shown only when the user is viewing the cart, is logged in, and the cart has contents. If either test fails, an error message is registered, and the script redirects to the cart/showcart.php script.

2. The inventory, items, and orders tables are locked for writing, and the users and customer tables are locked for reading. The inventory, items, and orders tables ...