Multiple WordPress galleries per page using attachment tags

tagged-galleries.php

<?php
/*
Plugin Name: Tagged Galleries
Description: Lets you add multiple [gallery] tags in a single WordPress page. Specify the images to be included in each gallery using tags: [gallery tag="foo"] (assuming you have tagged the images).
Version: 1.0
Author: Stephanie Leary
Author URI: http://sillybean.net/
*/

add_action('admin_init', 'register_attachment_tags');

function register_attachment_tags() {
	register_taxonomy_for_object_type('post_tag', 'attachment');
}

add_filter( 'post_gallery', 'tagged_gallery_shortcode', 9, 2);

function tagged_gallery_shortcode($output, $attr) {
	global $post;
	$ids = array();
	
	// if tag="foo" was included in $attr, transform it into a comma-separated list of post IDs
	if (isset($attr['tag'])) {
		$tag = $attr['tag'];
		
		if (term_exists($tag)) {
			$posts = get_posts( array('post_type' => 'attachment', 'posts_per_page' => -1, 'post_parent' => $post->ID, 'tax_query' => array(
					array(
						'taxonomy' => 'post_tag',
						'field' => 'slug',
						'terms' => $tag
					) )) );
			$ids = array();
			foreach ($posts as $apost) {
				$ids[] = $apost->ID;
			}
			$ids = implode(',', $ids);
		}
	}
	
	// now run the gallery shortcode as usual, using include=$ids in place of tag="foo"
	extract(shortcode_atts(array(
		'order'      => 'ASC',
		'orderby'    => 'menu_order ID',
		'id'         => $post->ID,
		'itemtag'    => 'dl',
		'icontag'    => 'dt',
		'captiontag' => 'dd',
		'columns'    => 3,
		'size'       => 'thumbnail',
		'include'    => '',
		'exclude'    => ''
	), $attr));

	if (!empty($ids))
		$include = $ids;
	
	// remove our filter so [gallery] will be processed as usual
	remove_filter( 'post_gallery', 'tagged_gallery_shortcode', 9, 2);
	$output = do_shortcode('[gallery 
		order="'.$order.'" 
		orderby = "'.$orderby.'" 
		id = "'.$id.'"
		itemtag = "'.$itemtag.'"
		icontag = "'.$icontag.'"
		captiontag = "'.$captiontag.'"
		columns = "'.$columns.'"
		size = "'.$size.'"
		include = "'.$include.'"
	]');
	// add our filter back so it's there the next time we encounter [gallery] in the content
	add_filter( 'post_gallery', 'tagged_gallery_shortcode', 9, 2);

	// return the gallery
	return $output;
}