会议为什么它没有注意到什么？在会话部分PHP

Question

我有这个代码。我认为没有错。但它不起作用。只需在mozilla上显示白屏。可能是，有人知道我的错误在哪里？请告诉我..

 <?php 
session_start();
include 'koneksi2.php';
$uname=$_POST['Name'];
$pass=$_POST['Password'];
$op=$_GET['op'];
//$pas=md5($pass);

if($op=='in'){
 $perintah = "select * from user WHERE username = '$uname' AND password = '$pass'";
 $hasil = mysql_query($perintah);
 if(mysql_num_rows($hasil)==1);
 {
 $c=mysql_fetch_array($hasil);
 $_SESSION['username']=$c['username'];
 $_SESSION['level'] = $c['level'];

  if($c['level']=="pemilik"){
  header("location:home_pemilik.php");
  }

  else if ($c['level']=="pelayan"){
  header("location:home.php");
  } 
 else{
    echo("password salah");
     }}
}else if ($op=="out")
    {
    unset($_SESSION['username']);
    unset($_SESSION['level']);
    header("location:login.php");
    } 
 ?>