How to return the id of the user who is in the session

require_once('conexao.php');
@$email = $_POST['email'];
@$senha = md5($_POST['senha']);
// $email = "[email protected]";
// $senha = "12346";

$pdo = $dbconn->prepare("SELECT userid, nome, nivel FROM usuario WHERE email=:email and senha=:senha");
$pdo->bindParam(":email", $email);
$pdo->bindParam(":senha", $senha);
$pdo->execute();
// print_r ($pdo);

$users = $pdo->fetchAll(PDO::FETCH_ASSOC);

if (count($users) <= 0)
{
    echo "<script>alert('Email ou senha errados');
                top.location.href='./index.php';
                </script>";
    exit;
}

// pega o primeiro usuário
$user = $users[0];

session_start();
$_SESSION['logged_in'] = true;
$_SESSION['userid'] = $user['userid'];
$_SESSION['username'] = $user['nome'];
$_SESSION['usernivel'] = $user['nivel'];
// print_r ($_SESSION);
// echo session_id();

header('Location: ./_link/link.php');

I want to return id on another page to make a insert in the bank

session_start();
  print_r($_SESSION);
if(!$_SESSION) {
    header("Location: .././index.php");
    exit;
  }