[jQuery] Decide then its error/success on ajax submit?

[jQuery] Decide then its error/success on ajax submit?


Hello
I use the validation plugin and the following code:
        submitHandler: function(form) {
            $('#submit').attr("disabled", "true");
            $.ajax({
                type: 'POST',
                url: 'submit.php',
                data: $('#registerform').serialize(),
                error: function() {
                    $('#mess').html("Failure");
                    $('#mess').css("display", "block");
                    alert('Fail!');
                },
                success: function(responseText) {
                    $('#mess').html("True");
                    $('#mess').css("display", "block");
                    alert(responseText);
}
            });
        }
submit.php is php script which checks the inputs for errors, but what
shall my php-script response if there is something wrong?
And can I have a "responsetext" with it? (like on success)
So the question is: who do I trigger the error function?