Creating one of two sessions php - php

Question

Im trying to create two different sessions. One for the admin, the other for normal users. If SQL returns admin_role as 1 = admin. If admin_role as 0 = user.
I have session_start() in the beginning of the document via include('connection.php'). When I press submit on my login form nothing really happens as for now.
if(empty($_POST["username"]) || empty($_POST["password"])){
$error = "Fill in both fields!";
} else {
// Define $username and $password
$username=$_POST['username'];
$password=$_POST['password'];
// To protect from MySQL injection
$username = stripslashes($username);
$password = stripslashes($password);
$username = mysqli_real_escape_string($db, $username);
$password = mysqli_real_escape_string($db, $password);
// Check username and password from database
$sql = "SELECT admin_role FROM user WHERE user_name='$username' AND user_password='$password'";
$result = mysqli_query($db, $sql);
$admin_role = $row['admin_role'];
// If username and password exist in our database then create a session.
// Otherwise echo error.
if(mysqli_num_rows($result) == 1){
if($admin_role == 1){
$_SESSION['admin'];
header("location: logged-in.php");
}
if($admin_role == 0){
$_SESSION['username'];
header("location: logged-in.php");
}
} else {
$error = "Wrong credentials";
}
}