Sebastian Bergmann has shared a presentation he originally gave at an Etsy event covering some of the history behind the popular PHPUnit tool and the development it's been through.

When Noah Sussman asked me to give a Code as Craft Technology Talk last week when I was consulting for Etsy I immediately said yes. [...] Just like with a talk that I gave last year, I suddenly had a chain of associations in my head that I just had to follow. And down the rabbit hole I went once more ...

He talks about the origins of PHPUnit (and mentions a few other tools), the move from PHP4 to PHP5, a change in version control from SVN to Git and features of the tool including mock objects and data providers. He also notes that not all tests are "good tests" and how, sometimes, backwards compatibility breaks are a good thing.