If you've worked with PHP at all, you'll realize that it works only after you upload it to your web server. You can make it work on your computer as well if you install an Apache server!
The PHP include function will make your life so much easier. You won't have to edit every single page if you want to make a change in a sidebar - you'll never go back!
This tutorial teaches you how to use the include() function to include a file from anywhere on your site - including from a subdomain or a directory. It does not use DOCUMENT_ROOT.