Sammy Kaye Powers has posted the latest parts in his series looking at testing the PHP language with phpt tests. So far he's helped you compile PHP from source, run the test suite, learn about the phpt files and debug failing tests. He continues the series with two new posts:
In the 5th part of his series he shows how to use the PHP gcov site to locate lines of code in the PHP language core that aren't tested yet, how to create a new test to cover it and generating a code coverage report to see how much you've tested. In Part 6 he shows you how to take what you've created and submit it back to the PHP project on GitHub as a Pull Request (no RFC needed) based on changes from your own forked repository.