Sebastian Bergmann has provided a pointer to the update Mike has posted concerning the Google Summer of Code project he's working on - implementing mutation testing in PHPUnit.

From Mike's update:

The most recent version committed to the mutation_testing branch is in a testable state. I would encourage anyone interested to try out the new mutation testing feature in this branch.

You can get complete information about the project - its methods and goals - from this page on the PHPUnit wiki. If you'd like to get this new code a try and see what his mutation testing is all about, here's a link to the subversion repository to grab the latest copy.