private function _convertOptions(array $options = [])
    {
        $converted = [];
        if (!empty($options)) {
            foreach ($options as $name => $value) {
                if (is_array($value)) {
                    if (!empty($value['AND'])) {
                        $converted[$name] = implode(self::PMP_AND, $value['AND']);
                    } else if (!empty($value['OR'])) {
                        $converted[$name] = implode(self::PMP_OR, $value['OR']);
                    } else {
                        $converted[$name] = ''; // bad params
                    }
                } elseif (is_bool($value)) {
                    $converted[$name] = $value ? 'true' : 'false';
                } else {
                    $converted[$name] = $value;
                }
            }
        }
        return $converted;
    }