setting the document root path in linux

Hi

I'm a newbie to php and I'm using

require_once($_SERVER['DOCUMENT_ROOT']."/includes/class.announcement.php");

to import pages in php. This works fine on a windows environment. However when I move this application to a linux environment the files dont get imported properly. This is because in a windows environment
$_SERVER['DOCUMENT_ROOT'] returns c:/xampp/htdocs etc.
but in a linux environment returns /home/servers etc.

How can this be rectified so that the path is properly picked up in a linux environment