PHP后端接收javascript通过XMLHttpRequest提交json数据

post的后端数据一直接收不到，折腾了好半天，还是格式不对，做个记录

function uploadData() {
    var data = JSON.stringify(arrayData, undefined, 2);
    var url = "http://XXXXXXXXXXXXXXX";
    xhr = new XMLHttpRequest();
    xhr.open("post", url, true);
    xhr.setRequestHeader('content-type', 'application/x-www-form-urlencoded');
    xhr.onload = uploadComplete;
    xhr.onerror = uploadFailed;
    xhr.send("data=" + data);
}

其中xhr.send("data=" + data) 要加一个变量名，不然php后端接收不到$_POST数据