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// shows all tags (e.g. in a posts category grouped alphabetically by parent tag)
function show_all_tags_by_parent() {
	// get all tags from current item
	$tags 	= get_tags(array('hide_empty'=>false, 'orderby'=> 'asc'));
	$parents 	= get_tags(array('hide_empty'=>false, 'parent'=>0));
	
	// loop through parents and echo children grouped by parent
	foreach ($parents as $key => $parent ) {
			
		echo '<h3 class="tag__parent">' . $parent->name . '</h3>';
			
		foreach ($tags as $key => $tag) :
			$tag_link = get_tag_link( $tag->term_id );
			
			if ($tag->parent == $parent->term_id) :
				if ($tag->parent !== 0) : 
					echo "<span class=\"label label-default\"><a href=\"{$tag_link}\" class=\"{$tag->slug}\">{$tag->name}</a></span> ";					
				endif;
			endif;
		endforeach; 
	}
}

function wd_hierarchical_tags_register() {

  // Maintain the built-in rewrite functionality of WordPress tags

  global $wp_rewrite;

  $rewrite =  array(
    'hierarchical'              => false, // Maintains tag permalink structure
    'slug'                      => get_option('tag_base') ? get_option('tag_base') : 'tag',
    'with_front'                => ! get_option('tag_base') || $wp_rewrite->using_index_permalinks(),
    'ep_mask'                   => EP_TAGS,
  );

  // Redefine tag labels (or leave them the same)

  $labels = array(
    'name'                       => _x( 'Tags', 'Taxonomy General Name', 'hierarchical_tags' ),
    'singular_name'              => _x( 'Tag', 'Taxonomy Singular Name', 'hierarchical_tags' ),
    'menu_name'                  => __( 'Taxonomy', 'hierarchical_tags' ),
    'all_items'                  => __( 'All Tags', 'hierarchical_tags' ),
    'parent_item'                => __( 'Parent Tag', 'hierarchical_tags' ),
    'parent_item_colon'          => __( 'Parent Tag:', 'hierarchical_tags' ),
    'new_item_name'              => __( 'New Tag Name', 'hierarchical_tags' ),
    'add_new_item'               => __( 'Add New Tag', 'hierarchical_tags' ),
    'edit_item'                  => __( 'Edit Tag', 'hierarchical_tags' ),
    'update_item'                => __( 'Update Tag', 'hierarchical_tags' ),
    'view_item'                  => __( 'View Tag', 'hierarchical_tags' ),
    'separate_items_with_commas' => __( 'Separate tags with commas', 'hierarchical_tags' ),
    'add_or_remove_items'        => __( 'Add or remove tags', 'hierarchical_tags' ),
    'choose_from_most_used'      => __( 'Choose from the most used', 'hierarchical_tags' ),
    'popular_items'              => __( 'Popular Tags', 'hierarchical_tags' ),
    'search_items'               => __( 'Search Tags', 'hierarchical_tags' ),
    'not_found'                  => __( 'Not Found', 'hierarchical_tags' ),
  );

  // Override structure of built-in WordPress tags

  register_taxonomy( 'post_tag', 'post', array(
    'hierarchical'              => true, // Was false, now set to true
    'query_var'                 => 'tag',
    'labels'                    => $labels,
    'rewrite'                   => $rewrite,
    'public'                    => true,
    'show_ui'                   => true,
    'show_admin_column'         => true,
    '_builtin'                  => true,
  ) );

}
add_action('init', 'wd_hierarchical_tags_register');

								<?php 

									$thisPostId 	= get_the_id();
									$tagsByTagParent = wp_get_post_tags($thisPostId, array('hide_empty'=>false,  'orderby'=> 'asc', 'parent'=>39));

									foreach ($tagsByTagParent as $key => $tag) :
										$tag_link = get_tag_link( $tag->term_id );

										echo "<span class=\"label label-default\"><a href=\"{$tag_link}\" class=\"{$tag->slug}\">{$tag->name}</a></span> ";					
									endforeach; 

								?>