PHPMailer::DKIM_Sign PHP Method

    public function DKIM_Sign($signHeader)
    {
        if (!defined('PKCS7_TEXT')) {
            if ($this->exceptions) {
                throw new phpmailerException($this->lang('extension_missing') . 'openssl');
            }
            return '';
        }
        $privKeyStr = !empty($this->DKIM_private_string) ? $this->DKIM_private_string : file_get_contents($this->DKIM_private);
        if ('' != $this->DKIM_passphrase) {
            $privKey = openssl_pkey_get_private($privKeyStr, $this->DKIM_passphrase);
        } else {
            $privKey = openssl_pkey_get_private($privKeyStr);
        }
        //Workaround for missing digest algorithms in old PHP & OpenSSL versions
        //@link http://stackoverflow.com/a/11117338/333340
        if (version_compare(PHP_VERSION, '5.3.0') >= 0 and in_array('sha256WithRSAEncryption', openssl_get_md_methods(true))) {
            if (openssl_sign($signHeader, $signature, $privKey, 'sha256WithRSAEncryption')) {
                openssl_pkey_free($privKey);
                return base64_encode($signature);
            }
        } else {
            $pinfo = openssl_pkey_get_details($privKey);
            $hash = hash('sha256', $signHeader);
            //'Magic' constant for SHA256 from RFC3447
            //@link https://tools.ietf.org/html/rfc3447#page-43
            $t = '3031300d060960864801650304020105000420' . $hash;
            $pslen = $pinfo['bits'] / 8 - (strlen($t) / 2 + 3);
            $eb = pack('H*', '0001' . str_repeat('FF', $pslen) . '00' . $t);
            if (openssl_private_encrypt($eb, $signature, $privKey, OPENSSL_NO_PADDING)) {
                openssl_pkey_free($privKey);
                return base64_encode($signature);
            }
        }
        openssl_pkey_free($privKey);
        return '';
    }