Learning Drupal 6 Module Development (book) - different questions will be ask'ed here

This is directly from the download code for goodreads.module:

<?php
// $Id$

/**
 * @file
 * Module for fetching data from GoodReads.com.
 * This module provides block content retrieved from a GoodReads.com
 * bookshelf.
 * @see http://www.goodreads.com
 */

/**
 * Implementation of hook_block
 */
function goodreads_block($op='list' , $delta=0, $edit=array()) {
  switch ($op) {
    case 'list':
      $blocks[0]['info'] = t('Goodreads Bookshelf');
      return $blocks;
    case 'view':
      $url = 'http://www.goodreads.com/review/list_rss/'
            .'398385'
            .'?shelf='
            .'history-of-philosophy';
      $blocks['subject'] = t('On the Bookshelf');
      $blocks['content'] = _goodreads_fetch_bookshelf($url);
      return $blocks;
  }
}

/**
 * Implementation of hook_help()
 */
function goodreads_help($path, $arg) {
  
  if ($path == 'admin/help#goodreads') {
    $txt = 'The Goodreads module uses the !goodreads_url XML '
      .'API to retrieve a list of books and display it as block '
      .'content.';
    $link = l('Goodreads.com', 'http://www.goodreads.com');
    $replace = array(
      '!goodreads_url' => $link
    );
    return '<p>'. t($txt, $replace) .'</p>';
    
  }
}

/* END HOOKS */


/**
 * Retrieve information from the Goodreads bookshelp XML API.
 * 
 * This makes an HTTP connection to the given URL, and retrieves
 * XML data, which it then attempts to format for display.
 *
 * @param $url URL to the goodreads bookshelf.
 * @param $num_items Number of items to include in results.
 * @return string String containing the bookshelf.
 */
function _goodreads_fetch_bookshelf($url, $num_items=3) {
  $http_result = drupal_http_request($url);
  if ($http_result->code == 200) {
      $doc = new DOMDocument();
      try {
        $doc->loadXML($http_result->data);
      } catch (DOMException $e) {
        $msg = "Error parsing bookshelf XML for %url: %msg.";
        $vars = array('%url'=>$url, '%msg'=>$e->getMessage());
        watchdog('goodreads', $msg, $vars, WATCHDOG_WARNING);
        return t("Getting the bookshelf resulted in an error."); 
      }
      
      return _goodreads_block_content($doc, $num_items);
      
    // Otherwise we don't have any data
  } else {
      $msg = 'No content from %url.';
      $vars = array( '%url' => $url );
      watchdog('goodreads', $msg, $vars, WATCHDOG_WARNING);
      return t("The bookshelf is not accessible.");
  }
}


/**
 * Generate the contents of a block from a DOM document.
 * Given a DOM document and the maximum number of entries,
 * generate some content.
 * @param $doc DOMDocument object containing Goodreads XML.
 * @param $num_items Number of items to format for display.
 * @return string Formatted string. 
 */
function _goodreads_block_content($doc, $num_items=3) {
  
  $items_nl = $doc->getElementsByTagName('item');
  $len = ($items_nl->length < $num_items) ? $items_nl->length : $num_items;
  
  $template = '<div class="goodreads-item">'
            .'<img src="%s"/><br/>%s<br/>by %s</div>';
  
  for ($i = 0; $i < $len; ++$i) {
    
    $item_ele = $items_nl->item($i);
    $children_nl = $item_ele->childNodes;
    
    // Default image: 'no cover'
    $img = 'http://www.goodreads.com/images/nocover-60x80.jpg';
    $link = 'http://www.goodreads.com';
    $author = '';
    $title = '';
    
    foreach ($children_nl as $child_node) {
      
      if ($child_node->nodeType == XML_ELEMENT_NODE) {
        switch ($child_node->nodeName) {
          
          case 'author_name':
            if (strlen($author) > 0) $author .= ', ';
            $author .= check_plain($child_node->textContent);
            break;
            
          case 'book_image_url':
            $img = check_plain($child_node->textContent);
            break;
            
          case 'title':
            $title = $child_node->textContent;
            break;
            
          case 'link':
            $link = check_plain($child_node->textContent);
            break;
        }
      }
    }
    
    $book_link = l($title, $link);
    $out .= sprintf( $template, $img, $book_link, $author);
  }
  $out .= '<div class="goodreads-more">'
         . l('Goodreads.com', 'http://www.goodreads.com')
         .'</div>';
  return $out;
}