PHP服务器无法使用ajax接收javascript对象

Hello i am trying to send fields from a from that i saved in a javascript object to a php server. I am using ajax but when i try to receive the object in php i get a length of 0 when debugging. Basically i can't receive the data. Please what did i do wrong.

Javascript code:
    //fields from from saved in an object.
    var obj={
      'user_name': username,
      'pwd': psswd1,
      'user_email': email,
      'user_phone': mobile,
      'sec_quest1': question1,
      'ans1': answer1,
      'sec_quest2': question2,
      'ans2': answer2,
      'user_address': address,
      'user_userInfo': user_info
  };

   console.log(obj);

   var data = JSON.stringify(obj)
   var xhttp = new XMLHttpRequest();

  xhttp.onreadystatechange = function() {
if (xhttp.readyState == 4 && xhttp.status == 200) {
//document.getElementById("txtHint").innerHTML = xhttp.responseText;
    alert(xhttp.responseText);
   }
 } 
 xhttp.open("POST", "server.php", true);
 xhttp.setRequestHeader("Content-type", "application/json");
 xhttp.send(data);
}

php code:

<?php
  $obj = json_decode($_POST["data"]);
 echo 'Name: '.sizeof($obj);
?>

It displays a size of zero meaning it's not receiving the data. Please what did i do wrong

You cant access the post variable when posting JSON to the server so you need to do this.

$str = file_get_contents('php://input'); //($_POST doesn't work here)
$response = json_decode($str, true);

Then retrieve the fields

$name = $response['user_name'];
$phone = $response['user_phone'];
// etc
// or just $response[0], $response[1], $response[2] etc etc

Basically the same answer as techblu3 above, but a little more detailed.

You might be posting raw data, that can be accessed in php in this way

    $obj = json_decode(file_get_contents("php://input"),true);
    // true parameter is used to decode as array
    // you can make it false to use object
    echo $obj["user_name"];

One solution would be serializing your data so you can send it with header x-www-form-urlencoded.

 serialize = function(obj) {
  var str = [];
  for (var p in obj)
   if (obj.hasOwnProperty(p)) {
    str.push(encodeURIComponent(p) + "=" + encodeURIComponent(obj[p]));
   }
  return str.join("&");
 }
 var obj = {
  'user_name': 'John Doe',
  'user_email': 'john@doe.net',
  'user_phone': '2122221111',
 };

  var params = serialize(obj);
  var url = "https://url.net";


  console.log(params);

  var xhr = new XMLHttpRequest();
  xhr.open("POST", url, true);

  //Send the proper header information along with the request
  xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

  xhr.send(params);

Your PHP file would would include

 <?php echo 'Name: '.sizeof($_POST); ?>