Forumlar / Cotonti / Support / use the url from last visited page

i have a form (contactus plugin) where i want the url from the last visted page someone came from to be added in a hidden field
now someone suggested me to track the sessions on each page with

 

$_SESSION['visited_pages'][] = $_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'].$_SERVER['QUERY_STRING'];

and then get the last page with

$prev_page = $_SESSION['visited_pages'][(count($_SESSION['visited_pages']-1)];

due to my lack of knowledge of php i dont know how to use it in the form.....

​if($cfg['plugin']['contactus']['subjects'] != ''){
    $cfgsubjects = explode(";", $cfg['plugin']['contactus']['subjects']);
    if (count($cfgsubjects) > 1 && !empty($cfgsubjects[1])){
        $cfgsubj = "<select name=\"subject\">\n";
        $iii=0;
        foreach($cfgsubjects as $x) {
            if (!empty($x)) {
                $subjects[$iii] = trim($x);
                if ($iii==$subject || (empty($subject) && $iii==0)) {
                    $cfgsubj .= "<option value=\"".$iii."\" selected=\"selected\">".$subjects[$iii]."</option>\n";
                } else {
                    $cfgsubj .= "<option value=\"".$iii."\">".$subjects[$iii]."</option>\n";
                }
                $iii++;
            }
        }
        $cfgsubj .= "</select>\n";
    }else{
        $cfgsubj = '';
    }
}else{
    $cfgsubj = "<input type=\"hidden\" maxlength=\"100\" size=\"24\" value=\"URL HERE\" name=\"subject\" id=\"subject\" class=\"text\" />";
}

or if someone has a better solution to do this?