php会话未设置或丢失

I can't access session data. In index.php I have this code and $_SESSION['token'] is not empty:

<?php
include "classes/session.php";
include "classes/token.php";
// include "classes/login.php";

use Biboletin\Session;
use Biboletin\Token;
Session::start();
var_dump($_SESSION);
?>

Down in the code in the same file i also have line

<input type="hidden" name="token" value="<?php echo Token::generate(); ?>" />

This is the method generate() In token.php:

public static function generate()
{
    Session::start();
    return Session::set('token', bin2hex(random_bytes(32)));
}

The action of the form is pointing to login.php where I var_dump($_SESSION) and there is nothing. I have Session::start() in all files.

And this is the method start():

public static function start() 
{ 
  if (!self::$instance) { 
    self::$instance = new self(); 
  }
  return self::$instance;
}