Sorry I don't work with IIS (life is too short) so I don't know exactly how that function works.
But you can get the physical location of the file using the variables:
$PHP_SELF - the location of the script under the document root.
$HTTP_SERVER_VARS["DOCUMENT_ROOT"] - the physical location of the document root directory.
or
$HTTP_SERVER_VARS["SCRIPT_FILENAME"] - this is the above two variables combined into one.