问AJAX POST返回空值

EN

function insertMessage() {
    var username = document.getElementById('username').value;
    var message = document.getElementById('message').value;
    var queryString = "username=" + username + "&message=" + message;

    // send the username and message information to be inserted into the database
    var url = 'classes/chatroom/chatroom.upload.php';

    // create xml request
    var request = createCORSRequest("POST", url)

    // create a function that will receive data sent from the server
    request.onreadystatechange = function () {
        if (request.readyState == 4 && request.status == 200) {
            alert(request.responseText);
        }
    }

    request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    request.send(queryString);
}

function createRequest(method, url) {
    var thisRequest = new XMLHttpRequest();

    if ("withCredentials" in thisRequest) {
        // thisRequest has 'withCredentials' property only if it supports CORS
        thisRequest.open(method, url, true);
    } else if (typeof XDomainRequest != "undefined") { // if IE use XDR
        thisRequest = new XDomainRequest();
        thisRequest.open(method, url);
    } else {
        thisRequest = null;
    }
    return thisRequest;
}

<?php
include 'chatroom.config.inc.php';  // the database file

$message_username = $_POST['username'];
$message_content = $_POST['message'];

if ($message_username == "Username: Once entered, you don't have to enter again" || $message_username == "") {
    $message_username = "Guest";
}

if ($message_content == "Message:" || $message_content == "") {}
else {
    $users->post_message($message_username, $message_content); // insert database row using PDO query
}
?>