Sammy Kaye Powers has a series of posts over on his site introducing you to testing the PHP language with .phpt tests. So far he's introduced the topic, shown how to run the tests and debugging failing tests.

If you've ever wanted to get involved with PHP internals, writing tests is a great way to get your foot into the door. The tests are written in PHP so you don't even need to know C to get started.

Each of the posts also comes with a screencast, narrated by Sammy, showing the information presented in the tutorial:

• 01: Compiling PHP from source
• 02: Running the test suite
• 03: All about .phpt files
• 04: Debugging failed tests

There's more to come in the series as he still plans to teach about how to fix current tests and how to eventually create your own. Stay tuned to his site for more tutorials in the series.