RSS feed based off of http://scribbles.clockworkcaracal.com/posts/2018-07-06-simple-php-webcomic-site/

feed.php

<?php

// Change these to suit your site!
$homepage  = "http://example.com"; // The home page of your comic
$comic_dir = "comics";             // The folder where comic image files are kept
$news_dir  = "news";               // The folder where post/news HTML files are kept
$digits    = 3;                    // The number of digits in each comic ID. e.g. 3 for 001

// Checks whether a page exists for a given ID in the directory
// If it does, returns the file name
function page_exists($idnum, $target_dir) {
  global $digits;
  if (strlen($idnum) < $digits) $idnum = str_pad($idnum, $digits, '0', STR_PAD_LEFT);
  if (filetype($target_dir) == 'dir') {
    $flist = glob("$target_dir/$idnum.*");
    if (count($flist) > 0) return $flist[0];
    else return false;
  }
  else return false;
}

function display_news($page_id) {
  global $news_dir;
  if ($news_file = page_exists($page_id, $news_dir)) include($news_file);
}

function add_item($item, $key) {
  global $comic_dir, $homepage;
  if (substr(mime_content_type("$comic_dir/$item"),0,5) == 'image') {
    $item_id = explode(".", $item)[0];
    echo "<item>
      <guid>$homepage/index.php?id=$item_id</guid>
      <title>$item_id</title>
      <link>$homepage/index.php?id=$item_id</link>
      <description><![CDATA[";
    
    display_news($item_id);
    
    echo "]]></description>
    </item>\n";
  }
}

function comics() {
  global $comic_dir;
  if (filetype($comic_dir) == 'dir') {
    $file_list = array_slice(scandir($comic_dir,1), 0, 20);
    array_walk($file_list, 'add_item');
  } else {
    echo filetype($comic_dir);
  }
}

 header( "Content-type: text/xml");

 echo "<?xml version='1.0' encoding='UTF-8'?>
 <rss version='2.0' xmlns:atom=\"http://www.w3.org/2005/Atom\">
 <channel>
 <title>The Garden of Everything</title>
 <link>$homepage</link>
 <atom:link href=\"$homepage/";

 echo basename($_SERVER['PHP_SELF']);

 echo "\" rel=\"self\" type=\"application/rss+xml\" />
 <description />
 <language>en-us</language>\n";

 comics();

 echo "</channel></rss>";
?>