Kirby Env use the vlucas/phpdotenv package and enable their features for Kirby. This package should be used if you want to store your project credentials or variables in a separate place from your code or if you want to have development and production access in different places.

composer require beebmx/kirby-env

You don't need to do anything if your want to access in any $page, just use the page method:

$page->env('VAR');

But if you want to set variables in your config.php file, first you need to load the object with:

\Beebmx\KirbyEnv::load('main/path');

You need to have an .env file in your main/path directory.
You can store any credentials or variables secure like:

KIRBY_DEBUG=false

SECRET_KEY=my_secret_key
PUBLIC_KEY=my_public_key

FOO=BAR
BAZ=${FOO}

When you create an instance of \Beebmx\KirbyEnv you need to load the environment with:

\Beebmx\KirbyEnv::load();

If you require the immutability provides by vlucas/phpdotenv, just:

\Beebmx\KirbyEnv::overload();

Here's an example of a configuration in config.phpfile:

<?php

\Beebmx\KirbyEnv::load('main/path');

return [
    'debug' => env('KIRBY_DEBUG', false),
    'SECRET' => env('SECRET_KEY'),
    'PUBLIC' => env('PUBLIC_KEY'),
];

It is important that you add to your .gitignore the .env file. The main/path is where the .env file is located.