urgent! cant get simple php and ajax to work together :(

[busby]

hey...im trying to display messages stored in a database on a page using ajax.

a user should choose from a drop down list the title of the message and then on selection that message should appear just below.

but i cant seem to do it and i dont know why there is no errors in what ive got so far.

here is the code for the drop down box where the user would select the title.

      <form>
        <select name="users" onchange="showMessage(this.value)">
        <option value="">Select a message:</option>
        <?php while($rows = mysql_fetch_array($results))
        {?>
        <option value="1"><?php echo $rows['heading']; ?></option>
        <?php } ?>
        </select>
        </form>
        <br />
        <div id="txtHint"><b>Message info will be listed here.</b></div>

the drop down is successfully populated with the correct titles.

here is the javascript in the HEAD of the same page:

<script type="text/javascript">
function showMessage(str)
{
if (str=="")
  {
  document.getElementById("txtHint").innerHTML="";
  return;
  }
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("txtHint").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","showmessage.php?id="+str,true);
xmlhttp.send();
}
</script>

and here is the php file which is used to display the message upon selection.

<?php
$id=$_GET["id"];

$cust = new customer;

$sql="SELECT * FROM messages WHERE m_id = '".$id."'";

$result = mysql_query($sql);
?>
<table border='1' cellspacing="4">
<?php
while($row = mysql_fetch_array($result))
  {
  echo "<tr>";
  echo "<th>Heading</th>";
  echo "<th>Message</th>";
  echo "</tr>";
  echo "<tr>";
  echo "<td>" . $row['heading'] . "</td>";
  echo "<td>" . $row['message'] . "</td>";
  echo "</tr>";
  }
?>
</table>

could someone please take a look at this and see whats wrong? ive spent so long on it. if you could tell me how to correct it it would be great

[sneakyimp]

When you try changing the select, does your function run? You could put an alert in there to find out:

alert('this function is working');

[bradgrafelman]

I think you need to find some basic AJAX tutorials; your Javascript function sends the request but never does anything with the response.

EDIT: Woops - disregard; apparently I can't read. :p

EDIT2: Your HTML and Javascript work fine for me, so it must be your PHP script that's failing to send the data back.

Have you tried viewing the PHP script directly from your browser (e.g. with a simulated 'id' value) to view its output?

Also, is that the entire PHP file? If so, you've got a lot of code missing (e.g. 'customer' class is undefined, you never connect to MySQL, etc.).