DevShed has a new article posted that, while not specific to PHP, can be adapted easily to Develop an Object Oriented Credit Card Transaction Processor

This article will walk readers through the process of outlining a flexible Object Oriented design that will facilitate adding Merchant Services and Payment Methods in the future without affecting the client code. Starting from the principles of the software, they make their case for an OOP approach, as well as other, more customer-centric issues (who, what, when, etc).

The examples they give are in C++, but can be easily adapted to PHP, given their large similarities (especially with PHP5). With the various private and public methods, PHP5's new class features will make for a happy translation...