control.php

<?php
  if (session_status() == PHP_SESSION_NONE) {
    session_start();
  }

  if (!isset($error))
  {
    $error = "";
  }

  if (isset($_SESSION["loggedIn"]))
  {
    $username = $_REQUEST["username"];
    $password = $_REQUEST["password"];
    $currentUser = getUserBySurnameAndPassword($username,$password)
    if ($currentUser)
    {
      $_SESSION["loggedIn"] = $currentUser;
      header('Location: login.php');
      die();
    }
    else
    {
      $error = "Login details do not match";
    }
  }
 ?>