The Zend Developer Zone has posted the next article in their OXID and Authorize.net series (part one) focusing on the setup and application code to accept credit card payments.

Over the next couple of pages, I'll discuss this in detail, showing you how to create an OXID eShop module to accept credit card payments via the AlertPay payment service. Unlike Authorize.Net, this service requires users to visit and enter credit card information into a third-party Web site. As such, integrating it with OXID eShop is somewhat more complex, but it can be done.

The tutorial includes not only the description of how to make it work but also sample code and screenshots to ensure it's easy to follow each step of the way. There's lots of content here and plenty of steps to follow.