Code coverage testing

Hi all. I've started poking/hacking around in the Zend source trying to implement code coverage statistics for php files. The end goal is to build php with a coverage option, place it on a test server, run test suites, then see which lines of code in which files were hit (more importantly which ones weren't). Very similar to gcov for those familar with gcc. Am I re-inventing the wheel? Does this already exist? Thanks, Mike