Passing javascript object to php file using ajax post method

Passing javascript object to php file using ajax post method

Iam desperately trying to pass a json object using ajax post method to a
php file, decode it and pass something back. Php's json_last_error
displays 4, which means Syntax error.
this.send = function()
{
var json = {"name" : "Darth Vader"};
xmlhttp=new XMLHttpRequest();
xmlhttp.open("POST","php/config.php",true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send("data="+json);
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById("result").innerHTML=xmlhttp.responseText;
};
};
};
<?php
if(isset($_POST["data"]))
{
$data = $_POST["data"];
$res = json_decode($data, true);
echo $data["name"];
}
?>