non so se è il tuo caso ma in giro per la rete ho trovato questo

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".