PHP and HTML includes allow you to edit HTML and PHP on one page and update every page that had the include. In Drupal, the path might change with different themes (example: sites/all/themes/yourtheme/includes/head.php)
With the $directory variable, you can easily call the path to your theme. You can create an includes folder, then add your include files in there.
Here's what your Drupal include call will look like: