Setting up a simple audiobook feed on a Synology NAS

<?php
// Audiobook RSS feed generator. For more details, see
// https://kzar.co.uk/blog/2022/08/16/setting-up-a-simple-audiobook-feed-on-a-synology-nas/

// Settings.
$feedName = 'Audiobooks';
$feedDesc = 'Audiobooks feed description';
$feedURL = 'http://192.168.1.100:60808';
$coverImageFileName = 'audiobooks.png';
$ffprobePath = '/volume1/@appstore/ffmpeg/bin/ffprobe';
$ffmpegPath = '/volume1/@appstore/ffmpeg/bin/ffmpeg';


function outputRSS($books) {
  global $feedName;
  global $feedDesc;
  global $feedURL;
  global $coverImageFileName;

  setlocale(LC_CTYPE, 'en_US.UTF-8');
  header('Content-type: text/xml');

  echo '<?xml version="1.0"?>' . "\n";
  echo '<rss version="2.0" xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd">' . "\n";
  echo '  <channel>' . "\n";
  echo '    <title>' . htmlspecialchars($feedName) . '</title>' . "\n";
  echo '    <link>' . htmlspecialchars($feedURL) . '</link>' . "\n";
  echo '    <description>' . htmlspecialchars($feedDesc) . '</description>' . "\n";
  echo '    <image>' . "\n";
  echo '      <url>' . htmlspecialchars($feedURL . $coverImageFileName) . '</url>' . "\n";
  echo '      <title>' . htmlspecialchars($feedName) . '</title>' . "\n";
  echo '      <link>' . htmlspecialchars($feedURL) . '</link>' . "\n";
  echo '    </image>' . "\n";

  foreach($books as $book_filename => $book_details) {
    echo '  <item>' . "\n";
    echo '    <title>' . htmlspecialchars($book_details['title']) . '</title>' . "\n";
    echo '    <itunes:title>' . htmlspecialchars($book_details['title']) . '</itunes:title>' . "\n";
    echo '    <description>' . htmlspecialchars($book_details['description']) . '</description>' . "\n";
    echo '    <author>' . htmlspecialchars($book_details['author']) . '</author>' . "\n";
    echo '    <pubDate>' . htmlspecialchars($book_details['published_date']) . '</pubDate>' . "\n";
    echo '    <link>' . htmlspecialchars($book_details['url']) . '</link>' . "\n";
    echo '    <guid>' . htmlspecialchars($book_details['url']) . '</guid>' . "\n";
    echo '    <enclosure url="' . htmlspecialchars($book_details['url']) . '" ' .
                        'length="' . htmlspecialchars($book_details['size']) . '" ' .
                        'type="audio/mpeg" />' . "\n";
    if (isset($book_details['cover'])) {
      echo '    <itunes:image href="' . htmlspecialchars($book_details['cover']) . '" />' . "\n";
    }
    echo '  </item>' . "\n";
  }

  echo ' </channel>' . "\n";
  echo '</rss>' . "\n";
}

function main() {
  global $feedURL;
  global $ffprobePath;
  global $ffmpegPath;

  // Read the cache file, if it exists.
  $cache_file = './.cache.json';
  $cached_book_details = array();
  if (file_exists($cache_file)) {
    $cached_book_details = json_decode(file_get_contents($cache_file), 2);
  }

  // Loop through each book (mp3/m4b) file.
  $book_details = array();
  foreach (glob('*.{m4b,M4B,mp3,MP3}', GLOB_BRACE) as $book_file) {
    $size = filesize($book_file);
    $file_name_without_extension = pathinfo($book_file, PATHINFO_FILENAME);

    // Reuse cached book details if available and the file size hasn't changed.
    if (isset($cached_book_details[$book_file]) &&
        isset($cached_book_details[$book_file]['size']) &&
        $cached_book_details[$book_file]['size'] == $size) {
      $book_details[$book_file] = $cached_book_details[$book_file];
      continue;
    }

    // Book details aren't already cached, so figure them out now.
    $details = array();

    // Read the book's ID3 tags (if any).
    $id3_tags = json_decode(shell_exec(
      escapeshellarg($ffprobePath) . ' ' .
      '-print_format json -show_entries stream=codec_name:format ' .
      '-select_streams a:0 -v quiet ' .
      escapeshellarg($book_file)
    ), true)['format']['tags'];

    // Take note of the book's title.
    if (isset($id3_tags['title'])) {
      $details['title'] = $id3_tags['title'];
    } else {
      $details['title'] = $file_name_without_extension;
    }

    // Take note of the book's author.
    if (isset($id3_tags['artist'])) {
      $details['author'] = $id3_tags['artist'];
    } else if (isset($id3_tags['composer'])) {
      $details['author'] = $id3_tags['composer'];
    } else {
      $details['author'] = 'Unknown';
    }

    // Take note of the book's description.
    // Note: Add extra newlines so that the description is easier to read in
    //       mobile podcast apps.
    if (isset($id3_tags['description'])) {
      $details['description'] = preg_replace("/\n+/", "\n\n",
                                             $id3_tags['description']);
    } else {
      $details['description'] = '';
    }

    // Append the author and narrator details to the description.
    $details['description'] .= "\n\n" . 'Author: ' . $details['author'] . "\n";
    if (isset($id3_tags['narrated_by'])) {
      $details['description'] .= 'Narrator: ' . $id3_tags['narrated_by'];
    }

    // Fetch the cover art (if any is available) from the file if the image is
    // not yet saved.
    $cover_file = $file_name_without_extension . '.jpg';
    if (!file_exists($cover_file)) {
      shell_exec(
          escapeshellarg($ffmpegPath) . ' -loglevel quiet -i ' .
          escapeshellarg($book_file) . ' -vcodec copy ' .
          escapeshellarg($cover_file)
      );
    }
    if (file_exists($cover_file)) {
      $details['cover'] = $feedURL . rawurlencode($cover_file);
    }

    // Take note of when the book was published.
    if (isset($id3_tags['year'])) {
      $details['published_date'] = date(DATE_RSS, strtotime($id3_tags['year']));
    } else {
      $details['published_date'] = date(DATE_RSS, filemtime($book_file));
    }

    // Take note of the book's other details.
    $details['url'] = $feedURL . rawurlencode($book_file);
    $details['size'] = $size;

    // Save the book's details to the associative array.
    $book_details[$book_file] = $details;
  }

  // Write all the book details to the cache file to save work next time.
  file_put_contents($cache_file, json_encode($book_details));

  // Finally, output the RSS!
  outputRSS($book_details);
}

// Ensure the feed URL ends with a '/' and that the ffmpeg paths look correct.
// Hopefully that should make configuration less error-prone.
if (substr($feedURL, -1) != '/') {
  $feedURL .= '/';
}
if (!file_exists($ffprobePath) || !file_exists($ffmpegPath)) {
  fwrite(STDERR, 'Incorrect $ffprobePath or $ffmpegPath.' . PHP_EOL);
  exit(1);
}

main();
?>