On PHPBuilder.com today there's a new tutorial showing you how to write and run unit tests with SimpleTest to test the interface of your site (forms), not just the backend code.

In addition to offer a complete suite of unit testing capabilities, SimpleTest is bundled with an internal Web browser which you can use to automate the navigation of your Web site and perform tests such as completing and submitting Web forms much in the same way a typical user would. In this tutorial, I'll show you how to use SimpleTest to automate these otherwise tedious tasks.

He starts with the basics of installing the tool and and running the sample test to ensure things are set up correctly. They include tests that show how to check for a successful form submission by checking things like response code and contents of the text on the result page.