Rookie Q: Simple includes…

I’m nearly finished with my first install and re-work of WordPress:
https://angledend.com/update
(content has been migrated from my old blog setup)

On the right-hand side there you can see a failed PHP reference. That’s where I’m attempting to drop in regular HTML code from another file, in the same way I would use Apache SSI’s on a regular site (note: I know nothing about PHP). I assumed I could just use this:

<?php get_emailaddress(); ?>

…to reference the code found in ’emailaddress.php’, a file that just contains the relevant HTML, but that obviously isn’t working. I see lots of information in these forums on more complex include functions, but none that tell me how to just drop a simple block of content into your page. Can anyone tell me how to do this or point me in the right direction?

Thanks!