var url = "login.php";
            var what = "LoginStatus(req.responseText)";

            function CheckLogin()
            {
                var username = document.getElementById("username").value;
                var password = document.getElementById("password").value;

                DoCallback("username="+username+"&password="+password);
            }

            function LoginStatus(Status)
            {
                if(Status == 0)
                    alert("Bad login!");
                else
                    alert("Login OK!");
            }

        </script>