Do you put code in WP functions.php? well don´t
Put it in a site plugin instead, functions file will change with the template at updates and probably make a mess if you forget to back it up.
So, you should either make a plugin, or just download our plugin, this plugin doesn’t contain anything more then a file to use instead of functions.php, you can easily enable disable as you wish.
Download RobChangs WordPress site plugin, use as you wish, add the plugin in you WP