不知怎的，我的XMLHttpRequest没有调用saveChat.php，只是刷新了onSubmit形式

Question

不工作的XMLHttpRequest函数部分：

 function save() {
 //the data from the form
 chat_message = document.getElementById("chat_message").value;
 var from_id = document.getElementById("from_id").value;
 var to_id = document.getElementById("to_id").value;
//the data string
var postdata = 'chat_message='+ chat_message + '&from_id=' + from_id +'&to_id=' + to_id;
 xmlhttp.open("POST","saveChat.php",true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
 xmlhttp.setRequestHeader("Content-length", postdata.length);
  xmlhttp.send(postdata);
      }

应该接收数据并插入数据库的saveChat.php文件：

 <?php 
    @session_start(); //start a session for to_id
     include 'dbconnect.php';
    $chat = strip_tags($_POST['chat']); //messages
    $from_id = $_POST['from_id'];
    $to_id = $_POST['to_id'];
    $timei = $_SESSION['time']; //time postes
    $datei = date("Y-m-d");  //date posted
    $nowId = 0; //now id
    $chatId = ''; //chat id
   //insert into ocmment box
   $chat_user = $db_con->prepare("insert into chat_message(chat_id,message,from_id,to_id,now_id,date,time)
   values(:chatId,:chat,:from_id,:to_id,:now_id,:datei,:timei)");
   $chat_user->execute(array('chatId' => $chatId, 'chat' => $chat,'from_id' => $from_id, 'to_id' => $to_id, 'now_id' => $nowId, 
   'datei' => $datei, 'timei' => $timei)); 
 //execute the comment
    ?>

假设插入消息的HTML表单：

<form name="inclusion" method="post" onSubmit="javascript: save();document.inclusion.action.focus();return false;">
 <div class="input-group">
    <input type="hidden"  name="from_id" id="from_id" value="<?php echo $_SESSION['user']; ?>" />
    <input type="hidden" name="to_id" id="to_id"  value="<?php if(isset($_REQUEST['ChatId'])) 
    echo $_REQUEST['ChatId']; else { $to_id = $db_con->prepare("select from_id from chat_message where to_id =".$_SESSION['user']); $to_id->execute(); $rowTOID = $to_id->fetch(); echo $rowTOID['from_id']; } ?>" />
      <input type="text" name="chat_message" id="chat_message"  placeholder="Type Message ..." class="form-control" size="50"/>
      <span class="input-group-btn">
      <input type="submit" value="Send" name="submit" class="btn btn-warning btn-flat"/>
      </span>
    </div>
  </form>