$query = "SELECT * FROM `users` WHERE username='$username'
                    and password='".md5($password)."'";

        $result = mysqli_query($con,$query) or die(mysql_error());
        $rows = mysqli_num_rows($result);

        if($rows==1)
        {
            $_SESSION['username'] = $username;

            header("Location: index.php");
        }
        else
        {
        }