I am a minimalist plugin which logs 'I am Foo!' when the plugins_loaded action fires.

foo.php

<?php
/**
 * Plugin Name: Foo
 * Description: I am a minimalist plugin which logs 'I am Foo!' when the plugins_loaded action fires.
 */

if ( !defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Hooked on the plugins_loaded action.
 */
function foo_plugins_loaded() {
	error_log( 'I am Foo!' );
}

add_action( 'plugins_loaded', 'foo_plugins_loaded' );

Code example for https://www.reddit.com/r/Wordpress/comments/16id6gu/comment/k0jfpy6/