If you are trying to install PHP under the default configuration of IIS7 with Windows 7 (and presumably Vista & Server 2008, but I observed it under Win7), you might run into problems (for example getting “Service Unavailable” errors). Here is how I managed to fix them:

First, make sure that you’ve installed all the “Application Development Features” as shown in the screenshot below. They are not installed by default and at least some of them are mandatory for being able to load PHP (I would assume that they are “CGI” and “ISAPI Filters” / “ISAPI Extensions”), but install them all, just to be on the safe side.

Now (from an Administrator command line) go to the “C:Windowssystem32inetsrvconfig” directory and open up the file “applicationHost.config” with notepad. Remove all the references to PHP. Finally, re-add the *php <-> php5isapi.dll script mapping. Make sure that all your apppools are started and now everything should work.

I got some of the advice from this blogpost: Where did my IIS7 server go? Troubleshooting 503 "service unavailable" errors. Picture taken from psd’s photostream with permission.