使用jQuery AJAX验证用户无法正常工作

I'm trying to authenticate a user using AJAX wrapped with jQuery to call a PHP script that queries a MySQL database. I'm NOT familiar with any of those technologies but I (sorta) managed to get them working individually, but I can't get the jQuery, AJAX and HTML to work properly.

[Edit:] I followed Trinh Hoang Nhu's advice and added a return false; statement to disable the Submit button. All previous errors fixed, I can't get the object returned by the AJAX right.

HTML

Here's the HTML snippet I use:

<form id="form" method='post'>
    <label>Username:</label>
    <input id="user" name="user" type="text" maxlength="30" required /> <br />
    <label>Password:</label>
    <input id="pass" name="pass" type="password" maxlength="30" required /> <br />
    <input id="url" name="url" type="text" value="<?php echo $_GET['url'] ?>" hidden />
    <input name="submit" type="submit" value="I'm done">
  </form>

jQuery/AJAX

Here's my jquery code for using AJAX to authenticate a user (sorry if the indenting is messed up because of the tabs):

function changeMessage(message) {
    document.getElementById("message").innerHTML = message; }

$(document).ready(function() {
    $("#form").submit(function() {
        changeMessage("Checking");
        //check the username exists or not from ajax
        $.post("/stufftothink/php/AJAX/login.php",
                    {user: $("#user").val(), pass: $("#pass").val(), url: $("#url") },
                    function(result) {
                        //if failed
                        if (result === 'false') {
                            changeMessage("Invalid username or password. Check for typos and try again");
                            $("#pass").val(""); }
                        //if authenticated
                        else {
                            changeMessage("Authenticated");
                            window.location.href = result; } 
                    } );
        //to disable the submit button
        return false;
    }   );
} )

PHP

And here's my PHP script that gets called:

<?php 
ob_start();
session_start();

$user = $_POST['user'];
$pass = md5($_POST['pass']);

mysql_connect('localhost', 'root', '');
mysql_select_db('stufftothink');

$query = "select * from users where user = '$user' and pass = '$pass'";
$result = mysql_query($query);

$i = 0;
while ($row = mysql_fetch_array($result)) {
  $i = 1; }

if ($i == 1) {
  $_SESSION['user'] = $user; 
    $invalid_urls = array('register.php', 'login.php');
    $url = $_REQUEST['url']; //not sure whether _GET or _POST
    if (in_array($url, $invalid_urls)) {
        echo '/stufftothink/profile.php'; }
    else {
    echo '/stufftothink/'.$url; }
    }

else {
  echo 'false'; }

mysql_close();
?>

Edit

I've been getting a lot of downvotes on this question. I had accidentally submitted the question without the explanation filled in. I went back to edit it, but when I came back, there were already 4 downvotes. It had barely been a couple of minutes. Am I doing something wrong, or were the first 5 minutes the problem?

First if you want to submit form using ajax, you must return false from your submit function. Otherwise your browser will handle it and redirect you to another page.

If you want to return an object from PHP, you must convert it to json using json_encode, for example:

//PHP
$return = array("url" => "http://www.google.com");
echo json_encode($return);
//would echo something like {"url":"http://www.google.com"}

//JS
$.post(url, data, function(data){
  alert(data.url);
});

You have no ending ;'s on functions.

Should be:

function changeMessage(message) {
    document.getElementById("message").innerHTML = message; 
};

$(document).ready(function() {
    $("#form").submit(function() {
        changeMessage("Checking");
        //check the username exists or not from ajax
        $.post("/stufftothink/php/AJAX/login.php",
               {user: $("#user").val(), pass:$("#pass").val() },
               function(result) {
              //if failed
              if (result === 'false') {
                        changeMessage("Invalid username or password. Check for typos and try again");
                    $("#pass").val("");
              }
              //if authenticated
              else {
                                changeMessage("Authenticatred");
                window.location.href = "/stufftothink/" +  result; 
              } 
       });
   });
});

Not sure if that'll fix it, but it's the only thing that jumps out at me.