On PHPBuilder.com today there's a new tutorial introducing PHPUnit, the popular unit testing framework (based off the ideas from JUnit). Jason shows how you can use it to make a more robust and easier to maintain application.

We are creating web applications, with one important difference between the two being that the latter are built using a rigorous process, and with that, a rigorous testing program. [...] The PHP community has long had a great testing utility at their disposal known as PHPUnit, and in this tutorial I offer a brief introduction to its benefits.

He shows how to get it installed (via PEAR), writing up a first unit test to check for the contents of a string (making HTML) and executing the test to verify the outcome. He aso mentions something interesting you can use when you need to send a lot of information over to a test - providers.