如何更新我的div?

I'm working in simple chat that only work in IE every thing is okay but don't know why is unable to update the div of "onlineMembers", getOnlineMembers.php

<?php
    include("connect.php");
    $query = "SELECT * FROM user WHERE state = 1 AND getp = 0";
    $result = $db->query($query);
    $num_result = $result->num_rows;

    $mems = "";
    for ($i = 0; $i < $num_result; $i++) {
        $row = $result->fetch_assoc();
        $mems = $row["userName"] . " " . $mems;
        $query = "update `user` set `getp` = 1 where `userName` = '". $row["userName"]. "'";
        $result = $db->prepare($query);
        $result->execute();
    }
    echo $mems; 
?>

index.php

<html>
    <head>
        <link rel="stylesheet" type="text/css" href="style.css">
        <script>

            function getOnlineMembers(pageName) {           
                var obj = new ActiveXObject("msxml2.xmlhttp");              
                obj.open("GET", pageName);
                obj.send();     
                var txt = obj.responseText;

                if (txt.length != 0)  {
                    var mems = txt.split(" ");              
                    for (var i = 0; i < mems.length - 1; i++) {
                        addOnlineMember(mems[i]);
                    }
                }

                setTimeout(function(){  getOnlineMembers(pageName); },5000);
            }
            function addOnlineMember(memberName) {          
                onlineMembers.innerHTML += "<div id='member' class='member'>" + memberName + "</div>";
            }
        </script>
    </head>
    <body onload='getOnlineMembers("http://localhost/Last/chat/getOnlineMembers.php");'>
        <div class="chatBox">
            <div class="onlineMembers" id = "onlineMembers">

            </div>
        </div>
    </body>
</html>

sorry for my bad english, and i hope that someoan help me