protected function sendHttpHeaders(): void
    {
        // grab content disposition
        $disposition = $this->opt->getContentDisposition();

        if ($this->output_name) {
            // Various different browsers dislike various characters here. Strip them all for safety.
            $safe_output = trim(str_replace(['"', "'", '\\', ';', "\n", "\r"], '', $this->output_name));

            // Check if we need to UTF-8 encode the filename
            $urlencoded = rawurlencode($safe_output);
            $disposition .= "; filename*=UTF-8''{$urlencoded}";
        }

        $headers = array(
            'Content-Type' => $this->opt->getContentType(),
            'Content-Disposition' => $disposition,
            'Pragma' => 'public',
            'Cache-Control' => 'public, must-revalidate',
            'Content-Transfer-Encoding' => 'binary'
        );

        $call = $this->opt->getHttpHeaderCallback();
        foreach ($headers as $key => $val) {
            $call("$key: $val");
        }
    }