PHPMailer\PHPMailer\SMTP::startTLS PHP Method

    public function startTLS()
    {
        if (!$this->sendCommand('STARTTLS', 'STARTTLS', 220)) {
            return false;
        }
        //Allow the best TLS version(s) we can
        $crypto_method = STREAM_CRYPTO_METHOD_TLS_CLIENT;
        //PHP 5.6.7 dropped inclusion of TLS 1.1 and 1.2 in STREAM_CRYPTO_METHOD_TLS_CLIENT
        //so add them back in manually if we can
        if (defined('STREAM_CRYPTO_METHOD_TLSv1_2_CLIENT')) {
            $crypto_method |= STREAM_CRYPTO_METHOD_TLSv1_2_CLIENT;
            $crypto_method |= STREAM_CRYPTO_METHOD_TLSv1_1_CLIENT;
        }
        // Begin encrypted connection
        if (!stream_socket_enable_crypto($this->smtp_conn, true, $crypto_method)) {
            return false;
        }
        return true;
    }