Including PHP files in your theme is simple. You just create a PHP file, upload it to  your theme folder, and then include it with the following code:

<?php getFile('YOURFILE.php'); ?>

Remember to replace "YOURFILE.php" with the actual name of your PHP file.

With the getFile function, the cascading theme functionality is in place. The framework will first check your primary theme for the existence of "YOURFILE.php". If it is not there, it will check the core theme. If the file is not in the core theme, nothing gets loaded. With the cascading theme functionality, you only have to define an include file in the core theme, and you can include it from any of the non-core themes.