مشکل در ایجاد صفحه لاگین

سلام دوستان عزیز

آقا من این کد لاگین به سایت رو نوشتم (البته از اینترنت گرفتم) ولی نمیدونم چرا با کلیک روی دکمه ورود، به سایت لاگین نمیکنه.

کد session هم درسته.

این کد لاگین:

 <?php
include('db-connect.php');
session_start(); // Starting Session
$error=''; // Variable To Store Error Message
if (isset($_POST['submit'])) {
if (empty($_POST['username']) || empty($_POST['password'])) {
    $error = "Username or Password is invalid";
}
else
{
// Define $username and $password
    $username=$_POST['username'];
    $password=$_POST['password'];
// Establishing Connection with Server by passing server_name, user_id and password as a parameter
// To protect MySQL injection for Security purpose
    $username = stripslashes($username);
    $password = stripslashes($password);
    $username = mysql_real_escape_string($username);
    $password = mysql_real_escape_string($password);
// Selecting Database
// SQL query to fetch information of registerd users and finds user match.
    $query = $conn->query("select * from login where password='$password' AND username='$username'");
    $rows = mysqli_num_rows($query);
    if ($rows == 1) {
        $_SESSION['login_user']=$username; // Initializing Session
        header("location: admin.php"); // Redirecting To Other Page
    } else {
        $error = "Username or Password is invalid";
    }
    $conn->close();        // Closing Connection
}
}
?>

این هم کد session:

 <?php
include('db-connect.php'); //connect to db
session_start();// Starting Session
// Storing Session
$user_check=$_SESSION['login_user'];
// SQL Query To Fetch Complete Information Of User
$ses_sql=$conn->query("select username from login where username='$user_check'");
$row = mysqli_fetch_assoc($ses_sql);
$login_session =$row['username'];
if(!isset($login_session)){
    $conn->close();    // Closing Connection
    header('Location: index.php'); // Redirecting To Home Page
}
?>