REST API中的会话原则

Question

I need to control a session in a REST API. My REST API is implemented with slim and doctrine and within the routes.php file I have defined the POST request of the login:

    session_start();

    $app->post('/login', function ($request, $response) {
    $em = getEntityManager();
        $args = $request->getParsedBody() ?? json_decode($request->getBody(), true);
        $user = $em->getRepository(Usuario::class)->findOneByUsername($args['username']);
        if (null == $user) {
            echo "<script language='javascript'>alert('User not found'); window.location='App.php'</script>" ;

        } else {
            if ($user->getPassword() == $args['password']){
                $_SESSION['id'] = $user->getId();
                $_SESSION['username'] = $user->getUsername();
                if($user->getAdmin() && $user->getEnabled()){
                    echo "<script language='javascript'>window.location='Admin.php'</script>" ;
                } else if($user->getEnabled()){
                    echo "<script language='javascript'>window.location='Comparator.php'</script>" ;
                } else {
                    echo "<script language='javascript'>alert('Account 
inabilited'); window.location='App.php'</script>" ;
                }

            } else {
                echo "<script language='javascript'>alert('Incorrect Password'); window.location='App.php'</script>" ;
            }
        }
    });

I have 3 page .php. In page App.php I have an access form. I want to do is that when any user wants to enter the Admin or Comparator page, he can not enter without logging in previously. For this I have added the following code to the pages previously named:

<?php if (session_status() != "PHP_SESSION_ACTIVE" && session_status() != 2) { ?>
         <script>window.location='App.php'</script>
    <?php }?>

But when entering a valid username and password I get the following error:

It's very weird because the field username in the class Usuario exists. Any solutions?