第一次访问时显示不同的页面

I found a snippet of code that redirects if it's the first visit, but when I tried to use it, it just stayed at that code. I don't really understand too much about the cookies and how it works, so maybe you can help! Here's the PHP code:

    <?php

    session_start();

    if (isset($_SESSION['FirstVisit'])) {

    $_SESSION['FirstVisit'] = 1;

    header("Location: http://example.com/index.php");

    // Don't forget to add http colon slash slash www dot before!

    }

?>

那么我该如何修复它，如果这是您第一次访问该网站，它会将您带到某个页面，如果不是，则是默认页面?

<?php
if (!isset($_COOKIE['firsttime']))
{
    setcookie("firsttime", "no", /* EXPIRE */);
    header('Location: first-time.php');
    exit();
}
else
{
    header('Location: site.php');
    exit();
}
?>