I had a problem with the REQUEST_URI server variable not being available in my PHP app when running under IIS on Windows. I followed these instructions to fix the problem: 100% Apache-Compliant REQUEST_URI for IIS and Windows.

I had to download and install ISAPI Rewrite 3 and Request_URI for IIS 1.1 which I installed into PHP by editing my php.ini file with this line:

 auto_prepend_file = C:\Program Files\PHP\request_uri.inc