$DOCUMENT_ROOT inoperable in html_wrap?

There are a few parts of my gallery.footer and other templates where I'd like to include other files in the template. I tried using $DOCUMENT_ROOT and $HTTP_HOST, but only literal values work.
Example: in gallery.footer, I have
<?php include ("http://zepfanman.com/boxfoot.php") ?>
Why won't the following work as an alternative?
<?php include ("$DOCUMENT_ROOT/boxfoot.php") ?>
I get the error:
Warning: main(/boxfoot.php): failed to open stream: No such file or directory in /homepages/1/d94604327/htdocs/gallery/html_wrap/gallery.footer on line 46

Warning: main(): Failed opening '/boxfoot.php' for inclusion (include_path='.:/usr/local/lib/php') in /homepages/1/d94604327/htdocs/gallery/html_wrap/gallery.footer on line 46

2:15EST EDIT (addition): I've noticed that $SCRIPT_URL within boxfoot.php displays as boxfoot.php (instead of the gallery page's url which boxfoot.php is included in). All the other pages on my website (non /gallery) display the actual page url instead of boxfoot.php. What am I misunderstanding about how the Gallery templates work?