var httpRequest;
if (window.XMLHttpRequest) {
    //IE7,Mozila,Safari...
    httpRequest = new XMLHttpRequest();
} else if (window.ActiveXObject) {
    //IE5,IE6
    //找出最新版MSXML剖析器
    var msxmls = ["MSXML2.XMLHttp.4.0","MSXML2.XMLHttp.3.0","MSXML2.XMLHttp","Mircosoft.XMLHttp"];
    for (i = 0; i < msxmls.length; i++) {
        try {
            //建立XMLHttpRequest物件
            httpRequest = new ActiveXObject(msxmls[i]);
            break;
        } catch (e) {
            // do something
        }
    }
}

var xmlHttpRequest = new XMLHttpRequest();

// init FormData
var formData = new FormData();
// add data
formData.append('user', 'Ted_Shiu');
formData.append('password', '123456');
// init XMLHttpRequest
var xmlHttpRequest = new XMLHttpRequest();
// build poet method and backend target
xmlHttpRequest.open('POST', 'ajax.php');
// send request
xmlHttpRequest.send(formData);

// obj is data want to post to backend
var obj = {
        'user': 'ted_shiu',
        'password': '123456'
    },
    jsonData;
// init XMLHttpRequest
var xmlHttpRequest = new XMLHttpRequest();
// build poet method and backend target
xmlHttpRequest.open('POST', 'json.php');
// build content type
xmlHttpRequest.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
// obj to url string
function formUrlEncode(obj) {
    var urlData = '';
    for (var x in obj) {
        urlData = urlData + x + '=' + obj[x] + '&';
    }
    urlData = urlData.substr(0, (urlData.length - 1));
    return urlData;
}
// send request
xmlHttpRequest.send(formUrlEncode(obj));
// if want ajax get data
// use xmlHttpRequest.send();
// xmlHttpRequest.send('user=ted_shiu&password=123456');

// ajax.php
<?php
    if (isset($_POST['user'])) {
        // do something
        if ($_POST['password']) {
            // do something
            $output = array(
                'status' => 'ok',
                'message' => 'msg'
            );
            // echo is reponse browser, must use json_encode()
            echo json_encode($output);
        }
    }
?>
// json.php
<?php
var_dump($_POST);
echo '<br>';
echo $_POST['user'];
echo '<br>';
echo $_POST['password'];
?>

xmlHttpRequest.onreadystatechange = function() {
    if (xmlHttpRequest.readyState == 4) {
        // do something
        if (xmlHttpRequest.status == 200) {
            alert(xmlHttpRequest.responseText);
            // parse response to object
            var data = JSON.parse(xmlHttpRequest.responseText);
            alert(data.status);
        }
    }
}