[Spinball]

Apologies, Abe (and others), but I need a bit of help, please.
I want to call the form with a passed forum number.
I have put the variable in thus:

PHP Code:

// Part 1
$vbulletin->input->clean_array_gpc('p', array(
'advert_type' => TYPE_STR,
'advert_title' => TYPE_STR,
'location' => TYPE_STR,
'confirmation' => TYPE_STR,
'f' => TYPE_STR
));

// Part 2
$advert_type = $vbulletin->GPC['advert_type'];
$advert_title = $vbulletin->GPC['advert_title'];
$location = $vbulletin->GPC['location'];
$confirmation = $vbulletin->GPC['confirmation'];
$f = $vbulletin->GPC['f']; 


and

PHP Code:

$formforumid = $f;
if (strlen($formforumid) == 0) {
   $errormessage = $vbulletin->userinfo['username'] . ", Fatal Error - please contact admin and tell them the formforumid variable was blank when trying to create a new classified advert.  Thanks!";
   eval('print_output("' . fetch_template('STANDARD_ERROR') . '");');
   exit();
} 


Yet when I call the script thus:
http://www.avforums.com/forums/newth...eds_form&f=115
I get the error message.
If I hard code the forum number in, it works perfectly.
Why is $f null?
Thanks in advance.