苗条的框架发送和接收json

Im just doing a simple POST, but for some reason no matter what I do, I cant read the JSON data

$.ajax({
        url: 'server/account/session',
        type: 'POST',
        data: { "username": name, "password": password} ,
        contentType: 'application/json; charset=utf-8',
        dataType: "json",
        success: function (data) {
            sessionStorage.accessToken = data["token"];
            sessionStorage.userid = data["userid"];
            alert ( sessionStorage.accessToken );
        },


        error: function () {

        }
    }); 

data: {"username": "chips", "password": "potato"}

my php:

$result = json_decode($app->request->getBody(), true);
var_dump($result);

would result in NULL. I don't get why it will return NULL, I sent a json then decode it in php into a. array, isnt that what Im doing?

QUESTION TWO: 2) A related question:

if I were to send through jquery.post

            jQuery.post("server/account/session", { "username": name, "password": password}, function(data){
                var result = JSON.parse(data); 

                sessionStorage.accessToken = result["token"];
                sessionStorage.userid = result["userid"];

                alert ( sessionStorage.accessToken );
            });

and recieve it in php by $userid = $app->request->params('username');

it works perfectly, but my question is, I thought when we do POST, the data is sent through the Body instead of header. How is it that when I do jquery.post, i can recieve it by app's request->params??

EDIT:

OK I fixed the problem. Turn out even if you specify content type to be Json. It doesnt mean it will convert the data to JSON. I had to do JSON.stringify on the data.

But can anyone help me on the second question?

Answer to question 2; You do not have to JSON.stringify() at all, when you use the right method:

var data = {
    username: user,
    password: pass
};
$.ajax({
    dataType: 'json',
    method: 'POST',
    url: 'server/account/session',
    data: data,
    success: function (data) {
      console.log(data);
    }
})
.always(function () {
  $('body')
    .removeClass('loading');
});

This should generate a proper JSON-Request which would be automatically parsed into $_POST, $_REQUEST by php in general and can also be understood by Slim.