[upthedale]

Check out the MSR project Pex (for .Net, not PHP however) http://research.microsoft.com/en-us/projects/pex/

It is designed to generate input for your code (the public surface by default) to determine whether it runs correctly. It can use code contract features in .Net to help inform whether your code is correct. It is great for finding those 'weird cases', and where you find a particularly interesting case, it is easy to add it as a test in your suite.

You can see it used (and play around yourself) through the browser here: http://www.pexforfun.com