Get Tags Associated with a Custom Taxonomy Term

get_tags_related_to_tax_term.php

<?php

/**
 *	Get Tags Related to Taxonomy Term
 *	This function was built out of necessity for something that
 *	would retrieve the most popular tags of a given term within
 *	a specific taxonomy that may or may not be custom.
 */

function get_tags_related_to_tax_term($taxonomy, $term) {
	
	// Make all of the args for Get Posts
	$get_posts_args = array (
		'posts_per_page' => -1,
		$taxonomy	 => $term,
	);
	
	// Get all posts that match the taxonomy and term
	$posts = get_posts($get_posts_args);
	$post_tags = array();
	
	// Loop through all posts
	foreach ($posts as $post) {
		
		// Queue Up Next Post Tag For Verification
		$post_tags_on_deck = wp_get_post_tags($post->ID);
		
		foreach ($post_tags_on_deck as $post_tag_on_deck) {
			if (!in_array($post_tag_on_deck, $post_tags)) {
				$post_tags[] = $post_tag_on_deck;
			}
		}
	}
	
	function sort_post_tags($a, $b) {
		if ($a->count == $b->count) {
	        return 0;
	    } else {
	    	return ($a->count < $b->count) ? 1 : -1;
		}
	}
	
	usort($post_tags, 'sort_post_tags');
	return $post_tags;
}


/**
 *	Get Tags Related to Taxonomy Term List Helper Function
 *	Essentially performs the same operation as get_the_tags_list() in Wordpress,
 *	however it uses the get_tags_related_to_tax_term() function to retrieve the tags
 *	to list.
 */
 
function get_tags_related_to_tax_term_list($taxonomy, $term, $num_requested) {
	$post_tags = array_slice(get_tags_related_to_tax_term($taxonomy, $term), 0, $num_requested);
	
	foreach($post_tags as $post_tag) {
		$format = '<a href="' . get_bloginfo('url') . '/tag/%s">%s</a>';
		echo sprintf($format, $post_tag->slug, $post_tag->name);
		echo (end($post_tags) !== $post_tag) ? ', ' : null;
	}
}

?>

It's not exactly perfect. The count used to sort the tags is the "global" post count. It would need to be tweaked if you wanted the count of only those posts within the taxonomy term. For our use, this was good enough.