Keep nav active on specific pages?

So I want it so when your on the https://www.sitename.com/about-jane-smith that the ABOUT link I have stays in the active state.

What I have so far for the navigation is this.
CSS:

.nav02{
	float:left;
	display:block;
	overflow:hidden;
	width:76px;
	height:119px;
	background:url(/wp-content/themes/child/images/nav01.png) no-repeat 0 0;
}

and then inc_nav.php which is an include from header.php
<a class="nav02" href="/about-jane-smith">

What I was attempting in my very limited PHP knowledge was this:

if ($firstURIvar == 'about') {
	$hover = "-119px";
}

But this is obviously causing me problems. Any help would be greatly appreciated.