I don't know about you, but getting PHP up and running on a random development machine can be a pain. It's not PHP's fault... it's often the size and complexity of the web server. PWS is a monstrous download (30 Mb last time I tried) and Apache, while smaller, is still bulky and can be quite picky when attempting to get PHP up and running.
Solution worth considering: the free BadBlue web server is tiny (about 60K), installs in seconds, supports PHP (the PHP help page is at http://badblue.com/helpphp.htm) and, yep, it's free.
Check it out at http://badblue.com if you need a quick solution to the web server problem.