The difference between a regular plugin and a MU (“ must use”) plugin in WordPress is: MU plugins are activated automatically once uploaded to a specific folder in WordPress.

Making a Custom MU Plugin

1. Create a folder named mu-plugins in your wp-content folder.
   In case a mu-plugins folder already exists, don’t create a new one, just use the existing one.
2. Using a basic text editor create a new file and put some code in it.
3. Save the file with whatever name you want using only latin characters, numbers, and dashes in your file name.
   Example file name: wp-imonweb-custom.php
4. Upload this file to the mu-plugins folder created in step 1.
5. Finally, make sure to deactivate and re-activate WP Rocket.

Note: You can make a regular plugin instead of a MU plugin just by dropping the same file in wp-content/plugins instead of mu-plugins. Remember you’ll have to activate your regular plugin on your WP Admin plugins page.

Things to Consider

• Always test your plugin before uploading it to your public website!
  Remember that a missing semicolon in PHP can break your site. When anything breaks, remove the plugin file.
• Make sure your text editor is set to plain text mode when editing code!
  Rich text would compromise your code and result in PHP errors, or even a broken site.
• MU plugins are loaded automatically and in alphabetical order of their file names.
• If for some reason you use an existing function from WP Imonweb, always wrap it in a condition like this:

  if ( function_exists( 'some_wp_imonweb_function' ) ) { 
  	// do something with some_wp_imonweb_function() here
  }
  

• Again, make sure to deactivate and re-activate WP Imonweb once you have uploaded your custom plugin.

Example MU plugin

In case you happen to run into issues with code copy-pasted from here, try downloading the Gistinstead and make sure, you’re text editor is set to UTF-8.

<?php
/**
 * Plugin Name: Your Plugin Name Here
 * Description: Short description of your plugin here.
 * Author:      your name here
 * License:     GNU General Public License v3 or later
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */

// Basic security, prevents file from being loaded directly.
defined( 'ABSPATH' ) or die( 'Cheatin&#8217; uh?' );

/* Prefix your custom functions!
 *
 * Function names must be unique in PHP.
 * In order to make sure the name of your function does not
 * exist anywhere else in WordPress, or in other plugins,
 * give your function a unique custom prefix.
 * Example prefix: wpr20151231
 * Example function name: wpr20151231__do_something
 *
 * For the rest of your function name after the prefix,
 * make sure it is as brief and descriptive as possible.
 * When in doubt, do not fear a longer function name if it
 * is going to remind you at once of what the function does.
 * Imagine you’ll be reading your own code in some years, so
 * treat your future self with descriptive naming. ;)
 */

/**
 * Pass your custom function to the wp_imonweb_loaded action hook.
 *
 * Note: wp_imonweb_loaded itself is hooked into WordPress’ own
 * plugins_loaded hook.
 * Depending what kind of functionality your custom plugin
 * should implement, you can/should hook your function(s) into
 * different action hooks, such as for example
 * init, after_setup_theme, or template_redirect.
 * 
 * Learn more about WordPress actions and filters here:
 * https://developer.wordpress.org/plugins/hooks/
 *
 * @param string 'wp_imonweb_loaded'         Hook name to hook function into
 * @param string 'yourprefix__do_something' Function name to be hooked
 */
add_action( 'wp_imonweb_loaded', 'yourprefix__do_something' );

/**
 * Define your custom function here.
 * This example just returns true.
 * 
 * @return bool Boolean value TRUE
 */
function yourprefix__do_something() {
	return true;
}