Probably not possible with current technology.
PHPUnit and SimpleTest both do a fine job of providing a framework for running tests. I suppose it's possible to write a test skeleton generator, something which given a class would write skeleton test methods to test it.
But to write a program clever enough to figure out what a method is supposed to do and then test the boundary conditions would take some doing. Be almost as easy to just have it write the code in the first place.
Don't know about SimpleTest but PHPUnit has some code coverage capability so you get some idea of how complete your tests are.