my-plugin.php

<?php
/*
Plugin Name: My Plugin
Plugin URI: https://stackoverflow.com/q/48890836/9217760
Description: Just a simple test plugin.
Version: 20180221.1
*/

// Either of these worked for me.
/*global $my_vars;
$my_vars = '1';*/
$GLOBALS['my_vars'] = '1';

// THIS IS REALLY JUST A TEST. =)
function my_plugin_option( $name ) {
    if ( 'my_vars' === $name ) {
        return '1';
    }
}

// You'll need to turn on WP_DEBUG and WP_DEBUG_LOG; then see wp-content/debug.log
// after plugin activation.
register_activation_hook( __FILE__, 'my_plugin_activation' );
function my_plugin_activation() {
    global $my_vars;

    error_log( var_export( $my_vars, true ) );  // NULL if $my_vars wasn't defined with "global" or $GLOBALS
    error_log( my_plugin_option( 'my_vars' ) ); // 1

    error_log( '"My Plugin" has been activated.' );
}

// You'll need to turn on WP_DEBUG and WP_DEBUG_LOG; then see wp-content/debug.log
// after plugin deactivation.
register_deactivation_hook( __FILE__, 'my_plugin_deactivation' );
function my_plugin_deactivation() {
    error_log( '"My Plugin" has been deactivated.' );
}