How to fix the menu button of the open page?

Question

PHP

When you hover over the menu, everything is OK, it is underlined. But when the page is opened, it is not fixed.

<!-- навигация -->
    <div class="main-menu">
      <?php require_once'inc/menu.inc.php'; ?>

    </div>

content of the plug-in menu:

<nav>
        <ul>
          <li><a href="index.php?id=home">главная</a></li>
          <li><a href="index.php?id=testing">тестирование</a></li>
          <li><a href="index.php?id=plans">план развития сайта</a></li>
        </ul>
      </nav>

.main-menu li a{
  text-decoration:none;
  text-transform:uppercase;
  color:white;
  font-size:110%;
}
.main-menu li a:hover,.main-menu li a:active, .main-menu li a:focus{
  color:#deeaff;
  padding-bottom:12px;
  border-bottom:5px solid #deeaff;
}

Answer 1

In php check if you are on this page then add class .active for example

if(url == current_page) {

}

Is the point clear?