Javascript/AJAX/PHP Help

Hi,

I currently have got a script from the interweb which retrieves data from a MySQL database using php, but does it via Javascript. See it here.

Heres the bit of script that im focussing on:

function showUser(str)
{
xmlhttp=GetXmlHttpObject();
if (xmlhttp==null)
 {
 alert ("Browser does not support HTTP Request");
 return;
 }
var url="inc/MostPopular.php";
url=url+"?q="+str;
url=url+"&sid="+Math.random();
xmlhttp.onreadystatechange=stateChanged;
xmlhttp.open("GET",url,true);
xmlhttp.send(null);
}

Currently it will only load data from the mysql database when the state of a drop-down box is changed. How do i get that script to fire when the function is called?

Thanks in advance.