Prevent form submit

I would like to set a js var to true but ONLY if the login data is correct, NOT IF the form is valid(fields empty .. ) how can i do that? I'm using the jQuery validation plugin and i modified the submitHandler, so that the form is only submitting, if flag is true.

var flag = false;

$('#login-form').validate(
{
    rules:
    {
        User: "required",
        Password: "required"
    },
    messages:
    {
        User: "Please enter a Username!<br />",
        Password: "Please enter a Password!",
    },
    submitHandler: function(form)
    {
        if(flag)
        {
            var message = document.createElement("div");
            message.id = "info";
            message.style.width = "250px";
            message.style.height = "25px";
            message.style.color = "green";
            message.innerHTML = '<p style="text-align: center;">Your Login Data is correct!.</p>';

            $("#login-form").after(message);
            form.submit();
        }
    },
});

Here's php part, where i would like to set flag to true, if the login data is correct, else it should stay as it is but how can i set flag now?

if(!empty($result))
{        
    **Here i want to set the Flag!!
    $flag = true; ??****

    $_SESSION["loggedin"] = $user;
    header ("Location: index.php?site=backend"); 
    exit;
}
else
{
    echo '<p style="text-align: center; color: red">The username or password is wrong!</p>';
    header ("Refresh:2; url=index.php?site=backend-login"); 
    exit;
}

or is there any better way to do it? I just would like to not always have the form reloaded, because it's a self-submitting form and then also show this information message.