Capturing a referral ID in a web form as a hidden field

I am trying to capture a referral ID from a URL into a web form as a hidden field.

For example the landing URL would be: http://www.yoursite.com/inquiry.html?a_aid=38561bce

I need the form on that page to capture something like:
<input name="lead_source" value="38561bce" type="hidden">

I have tried this Javascript code, but it just doesnt want to do anything:

<script>
function getQueryVariable(variable) {
var query = window.location.search.substring(1);
var vars = query.split("&");
for (var i=0;i<vars.length;i++) {
var pair = vars[i].split("=");
if (pair[0] == variable) {
return pair[1];
}
}
}

</script>
<script type="text/javascript" language="JavaScript"><!--
document.write('<input ');
document.write(' type="hidden" ');
document.write(' name="lead_source" ');
document.write(' value="' + getQueryVariable("a_aid") + '">');
//--></script>

HTML:

Does anybody have any other ideas? Thanks in advance.

 

Also, I dont need this to be in Javascript, it can use PHP or whatever, I just need it to work. Thanks; it's driving me crazy.

 

This should output what you are looking for:

<?php
echo "<input type=\"hidden\" name=\"lead_source\" value=\"".addslashes($_GET['a_aid'])."\">";
?>

PHP:

 

<input type="hidden" name="lead_source" value="<%=request.querystring("a_aid")%>" />

Code (Classic ASP):

Above is classic asp but would work in .net but isnt the preferred methodology for .net