$_SERVER['REQUEST_URI']', Apache 1.x/2.x IIS 5/6

Hi,

When I have a page not found in Apache I can have it redirected to a custom
../error.php file and I can use $_SERVER['REQUEST_URI'] to get the file that
was not found.

But as some of you might know, IIS does not have '$_SERVER['REQUEST_URI']'.

So what is a 100% secure way of finding the path of the requested uri using
IIS?

What I was thinking of doing is

if $_SERVER['REQUEST_URI'] exists the return it
else
if $_SERVER[QUERY_STRING'] exists strip the relevant info from it and return
the data
but how do I know for certain what the format will be? in my case,
(IIS6) I get
$_SERVER[QUERY_STRING'] =
404;http://www.example.com/unknown_page.php?var1=12&var2=14
but it that the case for all of them? shall I return anything after
'404;'?
else
return self?

I obviously did a search on google, but most example/work around just don't
work or don't achieve the expected result.

Many thanks.

Simon