Changes the WP /wp-admin url to /admin for for consistency and standardization purposes.

required-wp-admin-url.php

/**
 * Change the /wp-admin url to /admin
 *
 * Adapted from {@link https://gist.github.com/hakre/701245}
 */
class WPAdminUrl {

	/**
	 * The instance of the class
	 */
	static $instance;

	/**
	 * Bootstrap function
	 */
	static public function bootstrap() {
		null === self::$instance
			&& self::$instance = new self()
			;
		return self::$instance;
	}

	/**
	 * The cookie path has been set already (see config).
	 */
	public function __construct() {
		add_action('init', array($this, 'init'));
	}

	/**
	 * Filter the admin url
	 */
	public function init() {
		add_filter('admin_url', array($this, 'admin_url'), 10, 3);
	}

	/**
	 * Provide the admin url set in /config
	 */
	public function admin_url() {
		return SITE_ADMIN_URL . '/';
	}
}

return WPAdminUrl::bootstrap();