Kévin Gomez has a recent post to his site sharing some of the knowledge he gained when digging into Humbug, a mutation testing tool for PHP development.

While I’ve already used Humbug a few time, a recent article made my realise that I didn’t really know how it worked.

That’s when I got the idea to dig into Humbug to learn how it works, and publish my findings here.

He starts with a brief overview of Humbug for those not familiar with it - a mutation testing tool that reviews your unit tests to see how well they actually cover your code. It performs various operations (mutations) on the tests and evaluates the response. He then gets into how Humbug does this and what tools it uses to break down and understand your tests. He then goes through the actual code of the tool, walking through the tests, tokenizing the code and performing small changes to re-test and see how the results differ from the original results.