public function substring ()
    {
        $start  = $this->getParameter('start');
        $length = $this->getParameter('length');

        if (empty($start))
            $start = 0;

        if (empty($length))
            $length = strlen($this->getParameter('value')) - (int)$start;

        $str = substr($this->getParameter('value'), (int)$start, (int)$length);

        return $str;
    }