xmlhttp响应文本

I am trying to update a particular div of my web page without reloading the complete page. Instead on response complete web page gets inserted in the targetid. I am using a javascript function to do so. Here are my code of php from where javascript function is called.

if($check_like==0)
                        {
                            // if user has not liked this post
                            $divid_like = $postid.'like';
                            echo "<div id='$divid_like'><a href='#' onClick=\"ajax_post('Ajax.php?action=Like-Status','postid=$postid','$divid_like',0);\" name='Like'> Like </a> </div>";
                            //echo "<a href='action.php?action=Like-Status&postid=$postid' name='Unlike'> Like </a>";
                            echo "<a href='#' onClick='' > Comment </a>";
                            echo "<a href='#' onClick='' > Share </a>";
                        }

Here is my javascript function :

<script language="JavaScript">

function ajax_post(url , args , targetid, add)
{
    var xmlhttp;
    if(window.XMLHttpRequest)
    {
        // works for all browser abve IE7, chrome and firefox
        xmlhttp = new XMLHttpRequest();
    }
    else
    {
        //for IE5 and IE6 which i don't think i nedd to handle still
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }

    if(!xmlhttp)
        document.getElementById(targetid).innerHTML = "somesdlkvnoisnvoanfvnfnerroor";

    xmlhttp.open("POST",url, false);
    xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
    //xmlhttp.setRequestHeader("Content-length", args.length);
    //xmlhttp.setRequestHeader("Connection", "close");

    xmlhttp.onreadystatechange=function()
    {
        var xhr = event.target;
        if(xhr.readyState==4 && xhr.status==200)
        {
            if(add==0)
            {
                document.getElementById(targetid).innerHTML = xhr.responseText;
            }
            else
            {
                document.getElementById(targetid).innerHTML+= xhr.responseText;
            }
        }
        else
        {
            document.getElementById(targetid).innerHTML = "somebigbigbigrroor";
        }

    };

    xmlhttp.send(args);
}

Ajax.php file being called to process data:

<?php

include_once("Includes/master_login.inc");
include_once("Includes/connect_to_database.php");
include_once("Includes/Users.php");
include_once("Includes/Friends.php");
include_once("Includes/current_page_url.php");
include_once("JavaScript/XML.js");
include_once("profile_display.php");
include_once("Includes/Status_Likes_Comment.php");
include_once("Includes/message.php");
include_once("Includes/NewsFeed.php");
include_once("sendmessage.php");
$action = $_GET["action"];
$uid = $_SESSION['uid'];

//if(!user_online($uid)){$action=0;}

if($action=="Add-Friend")   { add_friend($uid,$_POST["uid"]); if($_POST["refresh"]="profile") echo display_profile($_POST["uid"]); }    
if($action=="Respond-Friend-Request")   {friend_accept($_POST["uid"],$uid); if($_POST["refresh"]="profile") echo display_profile($_POST["uid"]); }
if($action=="Like-Status")  { like_status($uid,$_POST["postid"]);   echo "Unlike"; }
if($action=="Chat-Send")    {add_message($uid,$_POST["to"],$_POST["message"]); echo display_message($uid,$_POST["to"]);}
//if($action=="Chat-Send") {echo $_POST["message"]; delay(1000);}
    ?>

On liking the status instead of being replaced by Unlike complete page gets loaded in this div.