How do I use html code in an .inc file which is called by a php file?

Title seems a little butchered, but basically I have a php file that is trying to call .inc file with includes code for a menu. I'm using:

<?php
    include_once("php_menu.inc")
?>

And this is my HTML code:

<nav>
        <ul>
        <li class="navigation"><a href="index.html">Home</a></li>
        <li class="navigation"><a href="topic.html">Information</a></li>
        <li class="navigation"><a href="quiz.html">Quiz</a></li>
        </ul>
</nav>

But how do I write this in php_menu.inc so that it's actually picked up?