private function buildFileList($directories = [], $fileList = [])
    {
        $output = [];
        foreach ($directories as $directory) {
            foreach ($fileList as $file) {
                $fileName = $directory . DIRECTORY_SEPARATOR . $file;

                if (is_file($fileName) && is_readable($fileName)) {
                    $output[] = $fileName;
                }
            }
        }

        return $output;
    }