public function cache($expires)
    {
        if ($expires === false) {
            $this->_headers['Expires'] = 'Mon, 26 Jul 1997 05:00:00 GMT';
            $this->_headers['Cache-Control'] = array(
                'no-store, no-cache, must-revalidate',
                'post-check=0, pre-check=0',
                'max-age=0'
            );
            $this->_headers['Pragma'] = 'no-cache';
        } else {
            $expires = is_string($expires) ? strtotime($expires) : $expires;
            $this->_headers['Expires'] = gmdate('D, d M Y H:i:s', $expires).' GMT';
            $this->_headers['Cache-Control'] = 'max-age='.($expires - time());
        }
        return $this;
    }