Custom taxonomy mu-plugin

tax-emoji.php

<?php
/*
Plugin Name: emoji Custom Taxonomy
Plugin URI: http://www.vanpattenmedia.com/
Description: Custom taxonomy registration
Author: Van Patten Media
Version: 1.0
Author URI: http://www.vanpattenmedia.com/
*/

if ( !taxonomy_exists('emoji') ) {
	function register_emoji_taxonomy() {
		register_taxonomy (
			'emoji',
			array (
				'moment',
			),
			array (
				'hierarchical' => true,
				'label'        => 'Emoji',
				'show_ui'      => true,
				'query_var'    => true,
			)
		);
	}
	add_action('init', 'register_emoji_taxonomy');
}