PHPMailer\PHPMailer\PHPMailer::createHeader PHP Method

    public function createHeader()
    {
        $result = '';
        if ('' == $this->MessageDate) {
            $this->MessageDate = static::rfcDate();
        }
        $result .= $this->headerLine('Date', $this->MessageDate);
        // To be created automatically by mail()
        if ($this->SingleTo) {
            if ('mail' != $this->Mailer) {
                foreach ($this->to as $toaddr) {
                    $this->SingleToArray[] = $this->addrFormat($toaddr);
                }
            }
        } else {
            if (count($this->to) > 0) {
                if ('mail' != $this->Mailer) {
                    $result .= $this->addrAppend('To', $this->to);
                }
            } elseif (count($this->cc) == 0) {
                $result .= $this->headerLine('To', 'undisclosed-recipients:;');
            }
        }
        $result .= $this->addrAppend('From', [[trim($this->From), $this->FromName]]);
        // sendmail and mail() extract Cc from the header before sending
        if (count($this->cc) > 0) {
            $result .= $this->addrAppend('Cc', $this->cc);
        }
        // sendmail and mail() extract Bcc from the header before sending
        if (($this->Mailer == 'sendmail' or $this->Mailer == 'qmail' or $this->Mailer == 'mail') and count($this->bcc) > 0) {
            $result .= $this->addrAppend('Bcc', $this->bcc);
        }
        if (count($this->ReplyTo) > 0) {
            $result .= $this->addrAppend('Reply-To', $this->ReplyTo);
        }
        // mail() sets the subject itself
        if ('mail' != $this->Mailer) {
            $result .= $this->headerLine('Subject', $this->encodeHeader($this->secureHeader($this->Subject)));
        }
        if ('' != $this->MessageID and preg_match('/^<.*@.*>$/', $this->MessageID)) {
            $this->lastMessageID = $this->MessageID;
        } else {
            $this->lastMessageID = sprintf('<%s@%s>', $this->uniqueid, $this->serverHostname());
        }
        $result .= $this->headerLine('Message-ID', $this->lastMessageID);
        if (!is_null($this->Priority)) {
            $result .= $this->headerLine('X-Priority', $this->Priority);
        }
        if ('' == $this->XMailer) {
            $result .= $this->headerLine('X-Mailer', 'PHPMailer ' . self::VERSION . ' (https://github.com/PHPMailer/PHPMailer)');
        } else {
            $myXmailer = trim($this->XMailer);
            if ($myXmailer) {
                $result .= $this->headerLine('X-Mailer', $myXmailer);
            }
        }
        if ('' != $this->ConfirmReadingTo) {
            $result .= $this->headerLine('Disposition-Notification-To', '<' . $this->ConfirmReadingTo . '>');
        }
        // Add custom headers
        foreach ($this->CustomHeader as $header) {
            $result .= $this->headerLine(trim($header[0]), $this->encodeHeader(trim($header[1])));
        }
        if (!$this->sign_key_file) {
            $result .= $this->headerLine('MIME-Version', '1.0');
            $result .= $this->getMailMIME();
        }
        return $result;
    }