If the path to your PHP directory has spaces (e.g. "C:\Program Files\PHP") and IIS gives a 500+ error or responds with "The specified module could not be found." when you try to run a PHP script, try changing the ISAPI DLL's path in IIS (in the "Home Directory" tab, under "Configuration...") to the 8.3-equivalent path.
That is, use C:\PROGRA~1\PHP\php5isapi.dll instead of "C:\Program Files\PHP\php5isapi.dll".

 Copy/Pasted from here.