Drop this script in a folder full of JPEG photos to publish a photo feed

feed.php

<?php
/**
 * Place this file in a folder full of JPEG images to publish an RSS feed of the photos.
 * 
 *  -  Files are sorted latest-first by modification date.
 * 
 *  -  File names are used for captions, underscore is replaced with a space, extension
 *     is removed.
 *
 *  -  Requires PHP5
 *
 * License
 * 
 * Copyright (c) 2009, Walter Lee Davis <waltd@wdstudio.com>
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without 
 * modification, are permitted provided that the following conditions are met:
 * 
 *   -  Redistributions of source code must retain the above copyright notice, 
 *     this list of conditions and the following disclaimer.
 * 
 *   -  Redistributions in binary form must reproduce the above copyright 
 *     notice, this list of conditions and the following disclaimer in the 
 *     documentation and/or other materials provided with the distribution.
 * 
 *   -  Neither the name of Walter Davis Studio nor the names of its contributors 
 *     may be used to endorse or promote products derived from this 
 *     software without specific prior written permission.
 * 
 *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS 
 *   IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, 
 *   THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 
 *   PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR 
 *   CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, 
 *   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, 
 *   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR 
 *   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF 
 *   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING 
 *   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS 
 *   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * @author Walter Lee Davis
 */
//CONFIGURATION:
//external URL to the folder containing this file -- trailing slash needed
$url = 'http://example.com/photo_folder/';
//feed name & details
$title = 'My Fabulous Photos';
$subtitle = 'A fabulous collection of photos.';
$author = 'Richard Avedon';
$limit = 20; //only 20 photos

//read the directory
$dir = dirname(__FILE__);
$photos = array();
$latest = 0;
foreach(scandir($dir) as $file){
  if(preg_match('/^[^\.].+?\.jpe?g$/i', $file)){
    $timestamp = filemtime($dir . '/' . $file);
    $latest = ($timestamp > $latest) ? $timestamp : $latest;
    $photos[$timestamp] = $file;
  }
}
$latest = gmdate('Y-m-d\TH:i:s\Z',$latest);
//sort by date descending
krsort($photos);
//templates
$feed = '<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>{{title}}</title>
<updated>{{latest}}</updated>
<author><name>{{author}}</name></author>
<id>{{url}}</id>
<subtitle>
  {{subtitle}}
</subtitle>
<link rel="self" href="{{url}}feed.php"/>
{{entries}}
</feed>
';
$entry = '  <entry>
    <title>{{item_title}}</title>
    <id>{{id}}</id>
    <link rel="enclosure" type="image/jpeg" href="{{url}}{{photo}}" />
    <link rel="alternate" href="{{url}}{{photo}}" />
    <summary type="html">
      {{image_tag}}
    </summary>
    <published>{{published}}</published>
    <updated>{{published}}</updated>
  </entry>
';

/**
 * recursive variable replacement
 *
 * @param string $strText Input text including {{variable}} placeholders
 * @return string with all templates replaced with existing global vars
 * @author Walter Lee Davis
 */
function replace_vars($strText){
  $out = preg_replace_callback('/\{\{(.+?)\}\}/im','replace_vars_callback',$strText);
  if(strpos($out,'{{') !== false) {
    return replace_vars($out);
  }
  return $out;
}

function replace_vars_callback($arrMatches){
  $match = $arrMatches[1];
  if(isset($GLOBALS[$match])) return $GLOBALS[$match];
  //the spaces between the brackets here are Unicode Word Joiner characters (U+2060) 
  //not actual spaces. This keeps away the infinite loop.
  return '{⁠{' . $match . '}⁠}'; 
  //return $arrMatches[0];
}
//build the current set of entries as an array of strings
$entries = array();
$i = 0;
foreach($photos as $date => $photo){
  if($i < $limit){
    $published = gmdate('Y-m-d\TH:i:s\Z',$date);
    $id = $url . $photo;
    $item_title = preg_replace('/_/', ' ', preg_replace('/\.[a-zA-Z]{3,4}$/','',$photo));
    $image_tag = htmlspecialchars('<img src="' . $id . '" alt="' . $item_title . '" /><br /><br />');
    $entries[] = replace_vars($entry);
    $i++;
  }else{
    break;
  }
}
//concatenate into a single string
$entries = implode("\n",$entries);
//build the outer shell
$feed = replace_vars($feed);
//send the result
header('Content-type: application/xml');
print $feed;
?>