Can't find the server path when, in http.conf, using Alias and DirectoryIndex

I want to write a simple indexing script to display a directory full of photos as a gallery of thumbnails. (There are various solutions out there for this, but they're all a bit more complicated than I need). I've added a file in /etc/apache2/conf.d that looks like this: Alias /photos /home/public/photos <Directory "/home/public/photos">

    AllowOverride None
    Order allow,deny
    Allow from all

    DirectoryIndex /cgi-bin/index.php

</Directory> I use "Alias" so that I can leave the photos where they are and not have to move them to DocumentRoot. I use "DirectoryIndex" so that the script doesn't have to be in with the photos. My problem is that the running script seems to have no way to work out the photos are in /home/public/photos. $_SERVER[REQUEST_URI] is "/photos/", but I can't see how to derive the server path from that, since $_SERVER[DOCUMENT_ROOT] is "/srv/www/htdocs". $_SERVER[PHP_SELF] is "/cgi-bin/index.php", so no use either. How can I do this? Is there a way to interrogate the alias, or can I set a variable in the conf file that PHP can pick up? Cheers, Paul.