Problem with Randy Hoyt "Adding Active States" video -- underline persists, also white space around navigation icon

Having trouble with $section variable in PHP, sets underline to 'on' in navigation bar, but the underline persists when changing pages. Also, in the 'shirts' page alone, there is a white border behind the 'shirts' icon and the underline persists in 'contact'. However, in 'contact', it appears to work OK (white border and underline do NOT persists in 'shirts').

Here's the error that occurs on the 'shirts' page:

See how "contact" is still underlined and "shirts" has a white block around it?

But here's what it looks like on the 'contact' page (looks as if behaving normally):

Here's my header.php file relevant to this code:

<ul class="nav">
    <li class="shirts <?php if ($section == "shirts") {echo "on"; } ?>"><a href="shirts.php">Shirts</a></li>
        <li class="contact <?php if ($section == "contact") {echo "on"; } ?>"><a href="contact.php">Contact</a></li>
    <li class="cart <?php if ($section == "cart") {echo "on"; } ?>"><a href="cart.php">Shopping Cart</a></li>
</ul>

And here's the assignment of the variables in shirts.php and contact.php, respectively:

<?php 
//init page title before referencing in 'header.php'
$page_title = "These are shirts";
$section = "shirts";

include ('includes/header.php');
?>

<?php 
//init page title before referencing in 'header.php'
$page_title = "Contact Mike";
$section = "contact";

include ('includes/header.php');
?>

Really not sure what I did wrong!!

I'm getting a lot out of this lesson and I'd really like to implement this functionality in my own website, so if I could have some help I'd really appreciate it!

Thanks, Avery