I have this php server side include which works great on linux:

<?php
require_once($_SERVER['DOCUMENT_ROOT'].'/includes/header.php');
?>

I am trying to get it to work on a godaddy windows server (shared deluxe account) which does allow php, but I keep getting a "failed to open stream" error. Any suggestions on how to get this to work? Thanks in advance for any help.

try using a relative path

require_once('includes/header.php');

DOCUMENT_ROOT is an APACHE environment variable

APPL_PHYSICAL_PATH would be the IIS/Windows equivalent (if PHP on Windows supports that (never needed to test it)