[Lynne]

Sounds like something else is wrong with your code. You can see this right in the search page:

    $vbulletin->input->clean_array_gpc('r', array(
        'f'                     => TYPE_UINT,
        'days'       => TYPE_UINT,
        'exclude'    => TYPE_NOHTML,
        'include'    => TYPE_NOHTML,
        'showposts'  => TYPE_BOOL,
        'oldmethod'  => TYPE_BOOL,
        'sortby'     => TYPE_NOHTML,
        'noannounce' => TYPE_BOOL,
        'contenttype' => TYPE_NOHTML,
        'type' => TYPE_STR
    ));

So, 'f' is what you want.