javascript - Cannot get data in PHP from JQuery Ajax post with JSON -

i trying simple test learn jquery ajax post method , have setup little example on server. can't work.

my javascript follows:

javascript:

var json = {"num string":"2","num":3,"num":11,"num":2,"num":"?"}; $.ajax({         type: "post",         url: "samplejsonpost.php",         data: json, //data post server         contenttype: "application/json; charset=utf-8",         datatype: "json",         error: function (jqxhr, status, err) {             console.log("error " + err + " " + status + " " + json.stringify(jqxhr)); //log error          },         success: function (data, status, jqxhr) {                 console.log(json.stringify(data)); //log data returned               $("span").text(data);         }     }); }; 

and here php:

php:

<?php $data = $_post["data"]; echo json_decode($data); ?> 

however gives me following error:

error syntaxerror: unexpected end of input parsererror {"readystate":4,"responsetext":"","status":200,"statustext":"ok"}  

so me looks $data variable in php not getting json sending post, correct? if can me find wrong? have tried lots of other solutions without success must missing simple?

change into

var json = {"data":{"num string":"2","num":3,"num":11,"num":2,"num":"?"}};