I'm trying to get an external menu included in sidebox

Sidebox shows up ok, but I'm trying to have:

PHP Code:
<?PHP include("menu.php"); ?>


in there.

I've tried putting it in the
/language/english/CUSTOM/sideboxes/menu_sidebox_defines.php

if I put plain text in the contents section it shows, but not the PHP code. It should be trying to include it from the site html directory, right?