On the GoodPHPTutorials.com website there's a new tutorial showing how to get the Authorize.net API to talk back and forth between it and your application.
I have taken the time to abstract the AIM API into my own class that not only simplifies the code we will need to work with that API but also makes it modular and easy to port from application to application. To begin you will need to download the AuthnetAIM class. You can find it here.
He walks you through the setup and configuration of the library and includes a few examples of how to make both simple and more complex transactions through it.