ajax chat

hello i'm creating an ajax chat system and everything is working fine but the problem, i'm facing is that i can't replace some part of the message with the emotion kept in the array...

If i run only the emotion script then its working fine but i can't immplement with the chat system..

Any body plz help...

<?php
ob_start();
session_start();
?>
<style>
    body{
        background-color: grey;
    }
    #box{
        width: 326px;
        height: 432px;
        border: 1px solid activeborder;
        margin: 0px auto;
    }
    #box1{
        width: 326px;
        height: 400px;
        background-color: #eee;
        margin-bottom: 10px;
    }
</style>
<form method="post" action="">
<div id="box">
    <div id="box1">
        <?php
mysql_connect("localhost","root","") or die(mysql_error());
mysql_select_db(chat) or die(mysql_error());
$req = mysql_query("SELECT * FROM msg") or die(mysql_error());
while($row = mysql_fetch_array($req)){
    $m = $row['message'];

echo "<div id='message'> $m </div>";
}
?>
    </div>
    <input type="text" id="chat" size="50"  />
    <input type="button" name="submit" value=" Chat " onclick="sendmessage()" />
</div>
</form>
<script>

function sendmessage()
{    
    var msg = document.getElementById("chat").value;    
    var xmlhttp;
    
    if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("box1").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","chat_msg.php?m="+msg,true);
xmlhttp.send();
}

</script>

and the chat_msg.php code is

<?php
session_start();
mysql_connect("localhost","root","") or die(mysql_error());
mysql_select_db(chat) or die(mysql_error());
$sess = session_id();
$mg = $_REQUEST['m'];

$sql = mysql_query("INSERT INTO msg VALUES('','$sess','$mg')") or die(mysql_error());

$req = mysql_query("SELECT * FROM msg") or die(mysql_error());
while($row = mysql_fetch_array($req)){
    $m = $row['message'];   

echo "<div id='message'> $m </div>";
 } 
?>

and the emotion code is

    $str = $_REQUEST['comment'];
    $emo = array("<3", "#12", "@153", "#45", "@352");
    $img = array("<img src='emotions/1.png' height='113' width='120' alt='ugly' />",
        "<img src='emotions/2.png' height='113' width='120' alt='happy' />",
        "<img src='emotions/3.png' height='113' width='120' alt='love' />", 
        "<img src='emotions/4.png' height='113' width='120' alt='sweet' />",
        "<img src='emotions/5.png' height='113' width='120' alt='smiley' />");
    $new_str = str_replace($emo, $img, $str);
    echo "<hr />";
    echo $new_str;
}

?>

i can;t figure where to put the emotion code so that it will work like facebook chat system.

Any help will be greatly appreciated... Thank u.