Handcoded ul/li navigation – detect and add current page class

So, I have hand coded my navigation to include a few extra links and exclude some pages, but mainly to make my links as images. here’s the simple unordered list;

<ul id="nav">
<li><a href="https://www.wormsigntshirts.co.uk/t-shirts/"><img src="https://www.wormsigntshirts.co.uk/images/nav/home.png"></a></li>
<li><a href="https://www.wormsigntshirts.co.uk/"><img src="https://www.wormsigntshirts.co.uk/images/nav/tshirts.png"></a></li>
<li><a href="https://www.wormsigntshirts.co.uk/faqs/"><img src="https://www.wormsigntshirts.co.uk/images/nav/faqs.png"></a></li>
<li><a href="https://www.wormsigntshirts.co.uk/contact-us/"><img src="https://www.wormsigntshirts.co.uk/images/nav/contactus.png"></a></li>
<li><a href="https://www.wormsigntshirts.co.uk/feed/"><img src="https://www.wormsigntshirts.co.uk/images/nav/rss.png"></a></li>
</ul>

What I want to know is how do I add the <li class=”current_page”> when actually on that menu item’s current page? is there a simple php echo i can add in there?