Selamat siang gan.. Mohon bantuanya

Gan kalau untuk kasus saya ini gimana ya? di sistem yang saya bikin kan ada level (admin, staff, marketing) masing" punya Jobdes yang beda, ketika Login berhasil halaman akan di direct ke folde masing" misal (admin/index.php, staff/index.php dst) coba di bantu Gan buat referensi TA saya.. saya udah coba ngikutin panduanya dari Internet tapi tetep belum bisa..

Script File :

index.php?
==============
<form method="post" action="cek_login.php">
<p><input type="text" name="username" placeholder="Username or Email"></p>
<p><input type="password" name="password" placeholder="Password"></p>
<p class="remember_me">
<label>
<input type="checkbox" name="remember_me" id="remember_me">
Remember me on this computer
</label>
</p>
<p class="submit"><input type="submit" value="Login"></p>
</form>

cek_login.php
=============
<?php
//Koneksi Ke Database

include "library/inc.koneksidb.php";
//----------------------------------------------

//Variable dari Form Login
$username = $_POST['username'];
$password = md5($_POST['password']);//Enkripsi Password

$sql = mysql_query ("SELECT * FROM users WHERE username ='$username' AND password ='$password'");
$data = mysql_num_rows ($sql);
$ada = mysql_fetch_array ($sql);


//Apabila Username dan Password ditemukan
if ($data > 0)
{
session_start();
//Menyimpan Username dan Level kedalam Sistem
$_SESSION ['id_user']=$ada['id_user'];
$_SESSION['username'] =$ada['username'];
$_SESSION['password'] =$ada['password'];
$_SESSION['nama_depan'] = $ada['nama_depan'];
$_SESSION['alamat'] = $ada['alamat'];
$_SESSION['email'] = $ada['email'];
$_SESSION['level'] = $ada['level'];


if($_SESSION['level'] == 'Administrator')
{
header('location:admin/index.php');
exit();
}
else if($_SESSION['level'] == 'Staff')
{
header('location:staff/index.php');
exit();
}
}

else {
echo" LOGIN GAGAL! SILAKAN COBA LAGI!";

}

?>