session_start();
if(!(isset($_SESSION['username'])) && !(isset($_SESSION['password']))){
        $_SESSION['username']=trim($_POST['username']);
        $_SESSION['password']=$_POST['password'];
}

$username = $_SESSION['username'];
$password = $_SESSION['password'];
if($username=="" || $password=="")
{
    echo "<script>
        alert('You need to fill all fields!');
        window.location.assign('reg.php');
        </script>";
}