Finding out the local path to a file.

I'm wondering if there's a way for PHP fo know the local path of REQUEST_URI. I've used a script to list directories, in order to replace the looks of the traditional Indexes look (apache). By setting DirectoryIndex to a fixed file (/list/lister.php) that file is now executed whenever there's no other indexfile in the folder. But how can I let it know which folder to work in? Let's say that it gets example.com/~simon/files/ as URL. All the local variables I've seen by using phpinfo() point to the location of the file, not the folder I'm requesting. Any ideas? //Simon