include path file errors

hi, my webroot is /a/b/current/ i am in /a/b/current/d/file.php file.php has a line require_once ($_SERVER[document_root]."/d/common.php"); common.php has a line which says require_once ($_SERVER[document_root]."/_common.php"); // main include file for whole site it sends me no errors about missing files, but the variables inside main include file _common.php are not set and don't exist in file.php where is my mistake? muchas gracias. -- my site <a href="http://www.myowndictionary.com">myowndictionary</a> was made to help students of many languages learn them faster.