从PHP获取Ajax POST请求中的数据？

我正在尝试使用Ajax发送POST请求,但是我无法获取PHP中发送的值.这是我的JavaScript代码：

$.ajax({
    url: "updatedata.PHP",
    type: 'post',
    data: JSON.stringify(jsonData),
    contentType: 'application/json',
    dataType: 'json',
    success: function(data, status, xhr)
    {
       //...
    }
});

我想用PHP访问数据.像这样的东西？

$data = $_POST['data'];

我的数据：

{"UID":"00a3b1b0-03b4-11e1-be50-0800200c9a66","Firstname":"Bastian","Lastname":"Sander","UserPenaltys":{"Penalty1":"110","Penalty10":"200","Penalty11":"210","Penalty12":"220","Penalty13":"230","Penalty14":"240","Penalty15":"250","Penalty16":"260","Penalty2":"120","Penalty3":"130","Penalty4":"140","Penalty5":"150","Penalty6":"160","Penalty7":"170","Penalty8":"180","Penalty9":"190"},"PenaltyCounter":16}

我试过这个：

$.post("updatedata.PHP", JSON.stringify(UserData), function (data) {
}, "json");

但$_POST [‘Firstname’]是空的……

解决方法:

为什么不使用$.post()？格式为：

$.post(<URI string>, <postdata object>, <handler>, <datatype>);

然后将数据像任何其他表单一样对待PHP(即使用$_POST变量).