Adding wordpress header to external site

How would I go about adding my wordpress header into an external site/template? The external site is in .tpl format. I’ve tried using

{php} include_once('path/to/my/theme/header.php');

nothing shows up. Even if I’m able to get the header to show up, is it possible to have wordpress functionality? I’ve looked at the codex page on adding wordpress to an external site, it didn’t help me.