How to change an include file depending on the page

How to change an include file depending on the page

I'm making a website and for all of the pages I've including header.php,
which is just a logo and navigation bar at the top of the page, below is
the code for the nav menu.
<nav id="navmenu">
<ul>
<li><a href="index.php" class="active">Home</a></li>
<li><a href="about.php">About</a></li>
<li><a href="portfolio.php">Portfolio</a></li>
<li><a href="contact.php">Contact</a></li>
</ul>
</nav>
How can I make it so that the class="active" changes depending on what
page the user is on?