Iterate on related posts

functions.php

function loop_related_posts( $posts_per_page = 3, $category = null, $post_ID = 0 ) {
  /****************************************************************************************************
   * Get the post related to the current-looping post's matching tags.
   * Creates its own loop.
   *
   * USAGE:
   *   <?php while ( loop_related_posts( 3 ) ) : ?>
   *     your standard template code
   *   <?php endwhile; ?>
   */
  global $wpdb;
  global $post;
  static $current_post = false;
  static $results = array();

  // ****** Terminate command.
  // Done on new loop to allow everything setup correctly in the last loop.
  if ( !$current_post === false && $current_post >= count( $results ) ) {
    $current_post = false;
    wp_reset_postdata();
    return $current_post;
  }

  if ( $current_post === false ) {

    // ****** Get current post data
    $post_ID = intval( $post_ID );
    if ( $post_ID == 0 ) {
      $post_ID = (int)$post->ID;
    }

    // ****** Cache
    $cachekey = 'relatedposts-' . $post_id . '-' . md5( $posts_per_page . "other" );
    $results = wp_cache_get( $cachekey, "theme-relatedposts" );

    // ****** No cache? Query!
    if ( $results === false ) {

      // ****** Set variables
      if ( $category ) $category = "AND tm.name = '{$category}'";

      $tags = get_the_tags( $post_ID );
      if ( $tags == false || $post_ID == 0 ) return false; // --> EXIT

      // ****** SQL Tags list
      $taglist = array();
      foreach ( $tags as $tag ) { $taglist[] = '"' . $tag->term_id . '"'; };
      $taglist = join( $taglist, ', ' );

      // ****** Toggle Extended Content Query
      $is_extended_content = '';
      if ( true ) $is_extended_content = ', p.post_content';

      $results = $wpdb->get_results(
        "SELECT DISTINCT COUNT(tr.object_id) AS rank, p.ID, p.post_title, p.post_excerpt, p.comment_count,  p.post_name, p.post_date {$is_extended_content}
        FROM {$wpdb->posts} AS p
          INNER JOIN {$wpdb->term_relationships} AS tr ON p.ID = tr.object_id
          INNER JOIN {$wpdb->term_taxonomy} AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id
          INNER JOIN {$wpdb->terms} AS tm ON tm.term_id = tt.term_id
        WHERE
          tt.taxonomy = 'post_tag'
          AND (tt.term_id IN ({$taglist}))
          AND p.post_status = 'publish'
          AND p.post_type = 'post'
          AND p.ID != {$post_ID}
          AND p.post_date < '" . current_time('mysql') . "'
        GROUP BY tr.object_id
        ORDER BY rank DESC
        LIMIT 0, {$posts_per_page}
      ");

      // ****** Cache
      wp_cache_set($cachekey, $results, "theme-relatedposts");
    }
    if ( !$results ) return false; // --> EXIT

    // ****** We have data, first loop is on!
    $current_post = 0;
  }

  if ( $current_post !== false ) { // This isn't 'else' because can be initialized just above.
    // ****** Loop
    $current_post++;
    $post = $results[$current_post - 1];
    setup_postdata( $post );
  }

  return $current_post;
}