require_once variable problems?

I hope someone can tell us what the problem here is, and hopefully how to fix it... require_once gives an error, but the exact same string in require, include, or include_once works! Is this an error in PHP, or is require_once not supposed to do this for some reason? <?php require_once("$DOCUMENT_ROOT/include/DatabaseBox.php"); //require("$DOCUMENT_ROOT/include/DatabaseBox.php"); //include_once("$DOCUMENT_ROOT/include/DatabaseBox.php"); //include("$DOCUMENT_ROOT/include/DatabaseBox.php"); ?> Parse error: parse error, expecting `T_CONSTANT_ENCAPSED_STRING' in /home/kdotson/pets/reqiretest.php on line 2 We tried putting everything into a variable before requiring it too, and that also failed. We're running PHP Version 4.0.2, and will download 4.0.3pl1, but we doubt that will fix it since we didn't see anything in the changelog about fixing a problem of this type. Thanks, Shawn