Since your goal is to create a simple HTML website, with static pages I don't think PHP is the best way to go.
You have two options:
Run PHP on your local computer to pre-process the files:
If you install PHP-cli (command line client), you can use it to process your PHP static pages. Redirect its output to a file and you have your desired output:
php-cli index.php > index.html
Use nanoc (ruby-based) to build your static website:
If you don't have a webserver with PHP enabled, I assume you do not have PHP as a requirement but rather found about <?php include('file') ?> while studying HTML.
With this in mind, I suggest you check out nanoc. It's a tool what uses ruby to help creating static HTML webpages, by providing ways to define a layout (what you're doing with PHP's include) and many other features.
It's quite simple to use and produces static HTML files that you can upload to any server or open with your browser directly and still enables many powerful features while developing your website.