为什么PHP会话不在页面之间保存变量？

Question

I am setting up an admin panel for a website, and everything was working fine on my local (MAMP) server. I uploaded the website to the server and the user authentication isn't working anymore. I am able to get a success from the server, but when I'm entering into a page, PHP can't find the required session variable, and thus redirects the user back to the sign in page.

I have tried on both PHP version 5 and 7. I have tried echoing the session variable upon verification. I have tried to simply store the variable on one page and reading it on another page in the same folder, and it didn't work as well.

page1.php

<?php
session_start();
$_SESSION["userid"] = 1;
?>
<a href="page2.php">To Page2</a>

page2.php

<?php
session_start();
if (!isset($_SESSION["userid"])) {
    header("Location: page1.php");
    die();
}

echo $_SESSION["userid"];

After I click the link in page1.php, page2.php redirects me to page1.php again without any error.

Answer 1

Is your website using some load balancer?

By default, PHP creates the session on server memory. So, if your website is using a different server for each request, the $_SESSION set values are lost. In this case, a good solution could be store sessions out of the server, maybe in a Redis or Memcached.