Ajax not working

[MDanz]

where did i go wrong? It should update mysql(insertsubscribe function) and change the image in the anchor tag.

php

$id= $row['id'];

echo "<div class='subscribe'><a id='s$id' href='javascript:subscribe($id);'><img src='/subscribe.jpg' alt='subscribe' /></a></div>";

ajax

function subscribe(number)
{
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("s"+number).innerHTML="<img src='/unsubscribe.jpg' alt='unsubscribe' />";
    }
  }
xmlhttp.open("GET","subscribe.php?id="+number,true);
xmlhttp.send();
}

subscribe.php

<?php  session_start();  
include "database.php";

$id = $_GET['id'];

$database = new Database();
$database->opendb();
$database->insertsubscribe($id);
$database->closedb();
   
?>