public function certonly($email, $webroot, $domains)
    {

        $result = $this->command . ' certonly --webroot --debug --agree-tos';
        $result .= ' --email ' . $email;
        $result .= ' -w ' . $webroot;

        foreach ($domains as $domain) {
            $result .= ' -d ' . $domain;
        }

        return $result;
    }