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>

Does anybody have any other ideas? Thanks in advance.

Also, I dont mind doing this in PHP or whatever else it's fine; doesnt need to be Javascript.

You can't "document.write" elements into forms
put the hidden input in the form and use

document.formname.hiddenElementName.value = getQueryVariable("key");

and at the basic PHP would be

<input type="hidden" name="whatever" value="<?$_GET['keyname']?>">

although you should "sanitise" the querystring value first