Some of you are like me: “frequent theme changer guy”. Here’s a trick I’ve learned to keep from having to re-write code every time I get bored with a theme: I use the “Must Use” Plugin. 

What’s an MU Plugin?

A “Must Use” Plugin is a perfect solution for setting up Custom Post Types, Taxonomies, Google Analytics, and other important – but not necessarily “updatable” code, that would otherwise be lost upon a theme change.

The general consensus is: CPT’s & Taxonomies should not be handled at the theme level, but rather, at the plugin level. 

An MU Plugin is code that cannot be deactivated or deleted unless you have sFTP access. Check out the history of Must Use Plugins and learn more at the WordPress Codex

Why not create a normal plugin or use functions.php?

You could. But there are extra steps to get it activated if you’re running a multi-site network. Furthermore, if others have access to the Plugins section of the WP Dashboard, there’s a chance it could be deleted or deactivated by mistake. Worse, if you load up important things in functions.php, You’ll lose everything if the theme changes.

By using an MU plugin, your code will always be “on”, no matter what network, theme, or curious guest blogger might do otherwise!

Ok, how do I create a “Must Use” Plugin?

1. You need to create a directory: /wp-content/mu-plugins/
2. Next, create a PHP file. You can name it anything you want. Mine is named grd_functions.php. The full path would look like:/wp-content/mu-plugins/grd_functions.php
3. Now add your code, save your file and upload

That’s it! Really!

How about an example?

Here is an example Must Use plugin:

<?php
/*
Plugin Name: Must Use Functions
Plugin URI: https://gregrickabyold.test
Description: This plugin contains important functionality that need to persist, even if the theme changes.
Version: 1.0.0
Author: Greg Rickaby
Author URI: https://gregrickabyold.test
*/

namespace GRD;

/**
 * Place Google Analytics in the header.
 *
 * @author Greg
 * @version 1.0.0
 */
function google_analytics() {
?>
<!-- Global site tag (gtag.js) - Google Analytics -->
<script async src="https://www.googletagmanager.com/gtag/js?id=UA-1212017-87"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'UA-123456-78');
</script>
<?php
}
add_action( 'wp_head', 'GRD\google_analytics', 9999 );