检索POST数据从Ajax调用PHP

三天过去了，仍然有问题得到这个工作的事情。 这Ajax调用我的JS文件似乎工作，当谈到发送JSON数据：

Three days had passed and still having problems to get this things work. This AJAX Call on my js file seems working when it comes to sending JSON data:

    var _lname = $('#ptLastName').val();
        var _fname = $('#ptFirstName').val();
        var _mname = $('#ptMiddleName').val();
        var _gender = $('#ptGender').val();
        var _bday = $('input[name="birthdate"]').val(); // $('#ptBirthDate').val();
        var _ssn = $('#ptSSN').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;
                        }

但在我的PHP文件：

but in my PHP file:

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

mysql_connect ("*****", "****") or die ('Error: ' . mysql_error());
mysql_select_db ("********");

$query = "SELECT Lastname, Firstname, MiddleName FROM tbl_people WHERE Lastname = '$lastname' || Firstname = '$firstname' || MiddleName = '$middlename'";

$result = mysql_query($query);

$row = mysql_fetch_array($result);

    if ($row) {     
        $response = json_encode(array('d' => true, 'test' => $lastname)); 
    }
    else { 
    $response = json_encode(array('d' => false, 'test' => $lastname));
    }
echo $response;
print json_encode($_POST);

从萤火控制台的一些错误说：

some error from firebug console says:

<br />
<b>Notice</b>:  Undefined index: lastName in <b>C:\xampp\htdocs\..\CheckPerson.php</b> on line <b>2</b><br />
<br />
<b>Notice</b>:  Undefined index: firstName in <b>C:\xampp\htdocs\..\CheckPerson.php</b> on line <b>3</b><br />
<br />
<b>Notice</b>:  Undefined index: middleName in <b>C:\xampp\htdocs\..\CheckPerson.php</b> on line <b>4</b><br />
{"d":false,"test":null}[]

我相信， json_de code（）工作正常在我的PHP文件，但 $ _ POST [''] 无法从我的Ajax调用识别我发布的数据W / C变量已宣布：

i believe that json_decode() is working fine in my php file but $_POST[''] can't recognize my posted data from my ajax call w/c variables had been declared:

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

我相信我做的事情与我的codeS似乎我已经在这里找到了很多问题，做什么，他们说的，但是不知道为什么发生错误。 是否有你所看到的任何问题/错误？请告诉我。

I believe I am doing right with my codes seems i had read many questions here and done what they had said but don't know why the error occurred. Is there any problem/bug you had seen? please tell me.