PHP session variable does not survive - php

Question

Pretty simple setup: When the page is loading, a random session token is generated and saved in $_SESSION["token"]. A Form contains this token in a hidden input field. No problems until this point. I submit the form to the very same page (action="") and then I try to check if the $_SESSION["token"] is equal to the token that has been sent via POST. A variable is changed accordingly, and then I generate a new session token that replaces the old $_SESSION["token"] before the page loads again.
Problem is: The $_SESSION["token"] is changed again the moment the page is called (Before I can compare both SESSION and POST tokens) Therefore, both tokens never match. And I can't figure out WHY it changes. It's not the lines of code I wrote, because these are executed aswell, replacing the random token of unknown origin once again, before the page loads.
INDEX:
<?php
session_start();
date_default_timezone_set("Europe/Berlin");
$BASE_URL = "http://" . $_SERVER['SERVER_NAME'] . $_SERVER['REQUEST_URI'];
$form_valid = false;
if (isset($_POST["token"]) && isset($_SESSION["token"])) {
if ($_POST["token"] == $_SESSION["token"]) {
$form_valid = true;
}
}
//Set (new) session token
$token = bin2hex(random_bytes(10));
$_SESSION["token"] = $token;
//Load actual page
include ("/backend/admin.php");
?>
INCLUDED PAGE:
<?php echo "FORM VALID:"; var_dump($form_valid); ?>
<?php if (!isset($_SESSION["admin"]) || !$_SESSION["admin"]) { ?>
<form id="verify" method="POST" action="">
<label>Password</label>
<input type="password" name="access">
<input type="hidden" name="token" value="<?= $_SESSION['token'] ?>">
<input type="submit" value="Senden">
</form>
<?php } else { ?>
...
<?php } ?>
any help is appreciated. thank you.

Answer 1

There was a problem in your logic, session token updates every time regardless the form is submitted is not,
$token = bin2hex(random_bytes(10));
$_SESSION["token"] = $token
Try this,
<?php
session_start();
date_default_timezone_set("Europe/Berlin");
$BASE_URL = "http://" . $_SERVER['SERVER_NAME'] . $_SERVER['REQUEST_URI'];
$form_valid = false;
if(empty($_SESSION["token"] )){
$_SESSION["token"]= bin2hex(random_bytes(10));
}
if (isset($_POST["token"]) && isset($_SESSION["token"]))
{
if ($_POST["token"] == $_SESSION["token"]) {
$form_valid = true;
unset($_SESSION["token"]);
}
}
include ("/backend/admin.php");
?>