[jQuery] A part of .ajax I don't understand

[jQuery] A part of .ajax I don't understand


Hi out there. I'm having a little trouble with my .ajax. Here's the
function:
var name = $('#username').val();
var pass = $('#pass').val();
$.ajax({
        url: 'signin.php',
        type: 'GET',
        data: 'username=' + name + '&pass=' + pass,
        success: function(submitreturn){
            alert(submitreturn);
        }
    });
Here's signin.php:
<?php
require_once("classes/Login.php");
$user = $_GET['username'];
$pass = $_GET['pass'];
$login = new Login();
if($login->login_user($user, md5(md5($pass) . POSTPASSKEY)))
{
        return "I DID IT";
}
else
{
        return "I DIDNT DO IT";
}
?>
Okay, so here's my question. When the .ajax runs, the alert window
contains the string that was in the username input, and <br />. Why
is this? Why isn't submitreturn the value that was returned by
signin.php? How can I get the value returned by signin.php?
Thanks for your help!
-Ethan