使用带有jQuery的XMLHttpRequest将JSON数据发送到PHP

我正在尝试使用 XMLHttpRequest对象从表单发送 JSON数据.我可以使用以下函数发送数据. FireBug中没有显示错误,请求中的JSON数据显示为FireBug.

但是,我将数据发送到echo.PHP,简单地返回内容：

<?PHP
print_r($_POST);
print_r($_GET);
foreach (getallheaders() as $name => $value) {
    echo "$name: $value\n";
}
echo file_get_contents('PHP://input');
?>

POST数组始终为空,但我可以看到file_get_contents返回的JSON字符串.这是怎么发生的？我究竟做错了什么？

echo.PHP的输出

Array
(
)
Array
(
)
Host: localhost
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:10.0.2) Gecko/20100101 Firefox/10.0.2
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: eo,de-de;q=0.8,de;q=0.6,en-us;q=0.4,en;q=0.2
Accept-Encoding: gzip,deflate
Connection: keep-alive
Content-Type: application/json; charset=utf-8
Referer: http://localhost/form.html
Content-Length: 88
Cookie: {{..to much data..}}
Pragma: no-cache
Cache-Control: no-cache
{"type":"my_type","comment":"commented"}

发送功能：

function submit(){
    var data={};
    data.type=document.form.type.value;
    data.comment=document.form.comment.value;

    //get right XMLHttpRequest object for current browsrer
    var x=ajaxFunction();

    var string = JSON.stringify(data);

    x.open('POST','echo.PHP',true);
    x.setRequestHeader('Content-type','application/json; charset=utf-8');
    x.setRequestHeader("Content-length",string.length);
    x.setRequestHeader("Connection","close");

    x.onreadystatechange = function(){
        if (x.readyState != 4) return;
        if (x.status != 200 && x.status != 304) {
            alert('HTTP error ' + req.status);
            return;
        }

        data.resp = JSON.parse(x.responseText);
        if(data.resp.status=='success'){
            alert('That worked!');
        }else{
            alert('That didn\'t work!');
        }
    }
    x.send(string);

    return false; //prevent native form submit
}

x.send('name1='+string+'&name2=value2');

xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded")