In another behavior-driven development related post on his blog, Padraic Brady talks about mutation testing and a library he's put together to support it in PHP - PHPMutagen.

As I said in my previous entry I was thinking about how to write a Mutation Testing engine. The "braindump" was to use PHP's built in Tokenizer to break down a class file into digestible pieces which could be mutated, and then reconstructed into a mutated file. Once you allow for a working copy of the original source (let's not mutate the original!) it turned out to be a reasonable approach.

He's not releasing it just yet ("don't get too excited until I find some time to scan through it again") but he does share some sample output from his test run - one instance where all is well and the other where one mutant "escapes" and causes an exception.