Another mentor for another Google Summer of Code project has been announced and Sebastian Bergmann will be mentoring Mike Lewis as he adds mutation testing for PHPUnit.

Sebastian quotes a definition of "mutation testing" as:

Mutation Testing, or Automated Error Seeding, is an approach where the testing tool makes some change to the tested code, runs the tests, and if the tests pass displays a message saying what it changed.

The key difference is that it makes it easier to run a series of tests rather than just one at a time, aiding in more complex testing abilities. Check out this page on the PHPUnit site for more information.