<?php

define('DB_URL', 'mysql://root:password@localhost:3306/database');
define('APP_URL', '[::1]');
define('APP_URL_PATH', '/path/to/application');
define('FOO', 'more things');

<?php

if (!is_file('settings.php')) {
    require 'settings.php';
    return;
}

define('DB_URL', getenv('DB_URL'));
define('APP_URL', getenv('APP_URL'));
define('APP_URL_PATH', getenv('APP_URL_PATH'));
define('FOO', getenv('FOO'));

<?php

require 'config.php';

$app = new SlimSlimApp();

// code

$app->run();