Why can't I get my JSON Data posted using AJAX in my PHP file?

Question

I have an AJAX script that post data in one of my PHP file:

     var _lname = $('#ptLastName').val();
    var _fname = $('#ptFirstName').val();
    var _mname = $('#ptMiddleName').val();
$.ajax({
                type: "POST",
                url: ".././CheckPerson.php",
                data: "{'lastName':'" + _lname + "','firstName':'" + _fname + "','middleName':'" + _mname + "'}",
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (response) {
                    var res = response.d;
                    if (res == true) {
                        jAlert('Person Name already exists!', 'Error');
                        return;
                    }

It works fine and I can see the JSON Data posted in the Firebug console. The problem is with this PHP code:

$firstname = json_decode($_POST['firstName']);
$lastname = json_decode($_POST['lastName']);
$middlename = json_decode($_POST['middleName']);
$response = array();

The above PHP code seems that it can't recognize the 'firstName','lastName', and 'middleName' as a posted JSON parameter, and return an Undefined index: firstName in C:... something like that for all the posted parameters.

I also tried using $data = $_POST['data'] and $_REQUEST['data'] to get all the JSON parameters and decode it using json_decode($data); but didn't work.

I've also used the AJAX shortened code for post $.post('.././CheckPerson.php', {data: dataString}, function(res){ });, it works great with my PHP file and my PHP file can now read lastName, firstName, and middleName, but i think it is not a JSON data but only a text data because firebug can't read it as JSON data. Now,i'm confused how will my PHP file read the JSON data parameters.Do you guys have any suggestions about this?

Answer 1

The problem is that dataType: "json" doesn't mean that you're posting json, but that you're expecting to receive json data from the server as a result of your request. You could change your post data to:

data: {myPostData : "{'lastName':'" + _lname + "','firstName':'" + _fname + "','middleName':'" + _mname + "'}"}

and then parse it on your server like

$myPostData = json_decode($_POST['myPostData']);
$firstname = $myPostData["firstName"];
$lastname = $myPostData["lastName"];
$middlename = $myPostData["middleName"];

Answer 2

One issue- you're using single quotes for your json. You should be using double quotes (according to spec).

{"lastName":"Smith", "firstName":"Joe"}

instead of 

{'lastName':'Smith', 'firstName':'Joe'}