If you're not already doing testing (like unit testing) in your PHP-based applications but want to start, this new post on Taha Shashtari's site might just be for you. It provides a "gentle introduction" to unit testing, what it is and some of the tools you can use to get started.
Learning testing can be very overwhelming especially if you're just starting out. [...] You might have read a bunch of PHPUnit tutorials and maybe you tried to apply some of what you've learned in your projects, but it doesn't always feel right. And sometimes you get into situations where you have no clue how to test some feature in your application.
If anything of this happened to you, don't feel bad, we all have had this experience. And it's almost because of the way we learn it.
He starts by talking about some of the main goals of testing in your application and the difference between manual and automatic testing. He then gets into each of the three main testing types and follows it with links (and summaries) to tools you can use to get started testing. He ends the post with a brief look at test-driven development and, setting the stage for the next tutorial in the series, looks at the path ahead.