private function addHeaderToOutput(array $data, string $output): array
    {
        if (isset($data['headers']) && \count($data['headers']) !== 0) {
            $output .= 'msgid ""' . $this->eol;
            $output .= 'msgstr ""' . $this->eol;

            foreach ($data['headers'] as $key => $value) {
                if (\is_array($value)) {
                    $first = true;

                    foreach ($value as $h) {
                        if ($first) {
                            $first = false;
                            $output .= \sprintf('"%s: %s"', $key, $h) . $this->eol;
                        } else {
                            $output .= \sprintf('"%s\n"', $h) . $this->eol;
                        }
                    }
                } else {
                    $output .= \sprintf('"%s: %s\n"', $key, $value) . $this->eol;
                }
            }

            unset($data['headers']);

            $output .= $this->eol;
        }

        return [$data, $output];
    }