this method gets the current page using php which will pass a word in this case active and places it inside the class parameter to set the page active.
<?php
function active($currect_page){
$url_array = explode("https://stackoverflow.com/", $_SERVER['REQUEST_URI']) ;
$url = end($url_array);
if($currect_page == $url){
echo 'active'; //class name in css
}
}
?>
<ul>
<li><a class="<?php active('page1.php');?>" href="http://localhost/page1.php">page1</a></li>
<li><a class="<?php active('page2.php');?>" href="http://localhost/page2.php">page2</a></li>
<li><a class="<?php active('page3.php');?>" href="http://localhost/page3.php">page3</a></li>
<li><a class="<?php active('page4.php');?>" href="http://localhost/page4.php">page4</a></li>
</ul>