Jquery Ajax call got json result 'undefined'

I want to get json result using Jquery. Now I always got 'undefined' result. I can get the json print out using

alert(data);

But always return 'undefined' using

alert(data.first_name);

Jquery client-side code

$.post(
    "/modules/services/userlogincheck_new.php",
    {
        dataType: 'jsonp',
        action : "checkpassword",
        email : email,
        password : password
    },
    function(data) {
        alert(data.first_name);
    }
);

PHP server-side

if ($_POST['action'] == "checkpassword") {
    $query = "select * from users where email='" . $email . "'";
    $result = mysql_query($query, $forumdbcon) or die('Error, insert query failed');

    while ($row = mysql_fetch_assoc($result)) {
        if (($row['password开发者_运维知识库'] == md5($password))) {
            $arr = array("response" => 1, "first_name" => $row['first_name'], "last_name" => $row['last_name'], "address" => $row['address1']);
            echo json_encode($arr);
        } else {
            $arr = array("response" => 2);
            echo json_encode($arr);
        }
    }
}

Try this:

$.post("/modules/services/userlogincheck_new.php", {
   action : "checkpassword",
   email : email,
   password : password
}, function(data) {
   alert(data.first_name);
},'json');

The response sent from the server is JSON not JSONP. Change dataType to JSON.

"Off topic": Your code is might be highly vulnerable to SQL Injection. If I transmitted ';DROP TABLE users; -- as my email address, you woud be in trouble.

On topic:

$.post(
    "/modules/services/userlogincheck_new.php",
    {
        action : "checkpassword",
        email : email,
        password : password
    },
    function(data) {
        alert(data.first_name);
    },
    'json'
);

$.post() needs the data-type to be set explicitely.

You don't need jsonp since this is on the server. Try changing dataType: 'jsonp' to dataType: 'json'.

You're using $.post, so you're just sending the dataType to PHP, which doesn't do anyting, you need to tell $.post to use JSON.

$.post("/modules/services/userlogincheck_new.php", {
  action : "checkpassword",
  email : email,
  password : password
}, function(data) {
  alert(data.first_name);
}, 'json;);

You're using jsonp, so you need to wrap your output in a function call, per the parameter given by the automatically created jsonp parameter.

echo $_GET['jsonp'] . '(' . json_encode($arr) . ');';

Alternatively, just use plain JSON instead.