Ajax not working?

where did i go wrong? It should update mysql(insertsuscribe function) and change the image in the anchor tag. This is my first time doing AJAX, what did i do wrong?

php

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

ajax

function suscribe(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.开发者_开发知识库onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
        document.getElementById("s"+number).innerHTML="<img src='/unsuscribe.jpg' alt='unsuscribe' />";
    }
  }
xmlhttp.open("GET","suscribe.php?id="+number,true);
xmlhttp.send();
}

suscribe.php

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

$id = $_GET['id'];

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

?>

Presumably the HMTL is valid. Ajax has issues if the html is not valid. Also if you are using firefox have you considered using a plugin such as https://addons.mozilla.org/en-US/firefox/addon/live-http-headers/ . This will help you identify if the issue is with the data not being sent, received or the page not updating.