<?php
    $links = [
        "welcome" => "Welcome",
        "cms" => "CMS",
        "contact" => "Contact"
    ];

    $currentPage = "welcome"; // default page

    if (isset($_GET["page"])) {
        $currentPage = $_GET["page"];
    }
?>
<nav>
    <ul>
        <?php
            foreach ($links as $link => $name) {
                $isActive = "";

                if ($currentPage == $link) {
                    $isActive = "active";
                }
                echo "<li><a href='index.php?page=$link' class='$isActive'>$name</a></li>";
            }
        ?>
    </ul>
</nav>