Best way to include a PHP file into the "Content" region from a custom module?

Best way to include a PHP file into the "Content" region from a custom module?
mymodule/includes/myphpfile.php
I'm clear on the hook_menu and page callback part. So it's really a question of how to output php into a page region from within the page callback function.

EDIT: I think I found my answer, invoking hook_theme and returning it through the page callback function. I'll leave this up since I'm still wondering what is the "best" solution.