private function _debug() {

        $result = '';

        // iterate through the defined constants
        foreach(get_defined_constants() as $name => $number)

            // iterate through the set options
            foreach ($this->options as $index => $value)

                // if this is a curl-related constant and it is one of the options that are set, add it to the result
                if (substr($name, 0, 7) == 'CURLOPT' && $number == $index) $result .= str_pad($index, 5, ' ', STR_PAD_LEFT) . ' ' . $name . ' => ' . var_export($value, true) . '<br>';

        // return the result
        return $result;

    }