WordPress independent functions.php

I have a fresh WordPress and bbPress installed on an internal server.

While I was setting up bbPress I wanted to test the functionalities like creating a forum, topic, etc. When doing these stuff in the back-end (dashboard) there didn't seem to be any problems but when I did it from the front-end I kept getting

ERROR: Are you sure you wanted to do that?

I searched for a solution and found this.

add_filter( 'bbp_verify_nonce_request_url', 'my_bbp_verify_nonce_request_url', 999, 1 );
function my_bbp_verify_nonce_request_url( $requested_url )
{
  return 'http://localhost:8888' . $_SERVER['REQUEST_URI'];
}

I simply changed the hard-coded URL to what our internal server is set up to and it fixed the problem.

Now my question: is it possible for me to add this solution to a functions.php that is independent of the theme being used? I asked this because I have 2 concerns:

The current theme might get updated and will overwrite the file

I'm aware that the solution to this is simply create a child theme but my second concern prevents me from doing this.

The WordPress administrator might change themes and so both the functions.php file on the main theme and the child theme will stop working

How could I add the solution above so that I don't have to worry about the theme being updated and/or replaced with a new theme in the future? I don't want to keep adding this solution every time the administrator decides to change themes.

Solution

If you can't put it in a theme, put it in a Plugin. If you're worried that the plugin will get de-activated make it a Must Use Plugin.

It's dead simple to create a plugin. Create a file plugin-name.php and place it in a directory wp-content/plugins/plugin-name/. That file should contain the following code:

<?php
/*
Plugin Name: Name Of The Plugin
Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates
Description: A brief description of the Plugin.
Version: The Plugin's Version Number, e.g.: 1.0
Author: Name Of The Plugin Author
Author URI: http://URI_Of_The_Plugin_Author
License: A "Slug" license name e.g. GPL2
*/

add_filter( 'bbp_verify_nonce_request_url', 'my_bbp_verify_nonce_request_url', 999, 1 );
function my_bbp_verify_nonce_request_url( $requested_url )
{
  return 'http://localhost:8888' . $_SERVER['REQUEST_URI'];
}

If you want it to be a must use plugin, put it in wp-content/mu-plugins/ instead of wp-content/plugins.