我有我的动态页面，如何在当前页面上添加活动类？

I have four php pages, homepage.inc.php, about.inc.php, contact.inc.php and heres my index.php

<div id="menu">
    <nav>
        <a id="home" target="_self" href="index.php">Home</a>
        <a id="about" target="_self" href="index.php?p=about">About us</a>
        <a id="contact" target="_self" href="index.php?p=contact">Contact</a>
    </nav>
</div>

<div id="content">
    <?php
    $pages_dir = 'pages';
    if (!empty($_GET['p'])) {
        $pages = scandir($pages_dir, 0);
        unset($pages[0], $pages[1]);
        $p=$_GET['p'];


        if(in_array($p.'.inc.php', $pages)){
            include($pages_dir.'/'.$p.'.inc.php');


        }else {
         echo 'Sorry, page not found.';
        }
    }else{
        include($pages_dir.'/home.inc.php');
        }
    ?>

</div>

this is my css active{background:gray;} I want to add an active class on my menu. How?

You can do something like this:

<a id="contact" target="_self" href="index.php?p=contact" <?php if ($_GET['p'] == "contact") {
    echo  'class="active"';
    } ?> >Contact</a>

Or using short hand PHP (making the code look cleaner)

<a id="contact" target="_self" href="index.php?p=contact"<? (($_GET['p']=="contact") ? 'class="active"' : '') ?>>Contact</a>

Or you can use some JavaScript with JQuery:

<script>
$('#<?php echo $_GET['p'] ?>').addClass('active');
</script>

change your code as shown below ">Home ">About us ">Contact

<?php
$homeCls = '';
$aboutCls = '';
$contactCls= '';
    $pages_dir = 'pages';
    if (!empty($_GET['p'])) {
        $pages = scandir($pages_dir, 0);
        unset($pages[0], $pages[1]);
        $p=$_GET['p'];

       if($p == '') {
     $homeCls = 'active';
       } else if($p == 'about'){
         $aboutCls = 'active';
       } else if($p == 'contact'){
         $contactCls= 'active';
       }
        if(in_array($p.'.inc.php', $pages)){
            include($pages_dir.'/'.$p.'.inc.php');


        }else {
         echo 'Sorry, page not found.';
        }
    }else{
        include($pages_dir.'/home.inc.php');
        }
    ?>