Determine current page in PHP

I somewhat finished the PHP development course and wanted to apply it in a personal project.

a href="portfolio.php" class="portfolio <?php if ($section == "portfolio") { echo "on"; } ?>">Portfolio</a

Here I am applying the trick Randy used in his Simple PHP application course.

echo "<pre>";
echo "portfolo: ";
var_dump($section == "portfolio");
echo "skillset: ";
var_dump($section == "skillset");
echo "about: ";
var_dump($section == "about");
echo "contact: ";
var_dump($section == "contact");

Here I checked the values and each boolean works as expected.

.<code> navigation ul li.on a { text-decoration: underline; color:white; } </code>

In the style.css I found Randy used this to react on the dynamic php selecting method or at least that is what I think he does.

I do not know the li.on (function?) in css. I question how css can determine that the list item is "on" merely on the text that is in the class of the anchor.

Does anyone have an idea on this issue? An explanation or link on this matter would be deeply appreciated.