Override a configuration dynamically in Drupal 8

In this post, I'll show you how to override a configuration dynamically in Drupal 8.

You can override configuration from settings.php files using the variable $config.

Examples: 

$config['system.maintenance']['message'] = 'Sorry, our site is down now.';

For nested values, use nested array keys

$config['system.performance']['css']['preprocess'] = 0;

Example of Overriding Payment Gateway configuration for security reasons:

$config['commerce_payment.commerce_payment_gateway.paypal_express_checkout']['configuration'] = [
    'api_username' => 'MY_API_USERNAME',
    'api_password' => 'MY_API_PASSWORD',
    'signature' => 'MY_SIGNATURE',
];

When using $config override outside of settings.php, use a preceding global $config;

To get the original values without override just use:

$message = \Drupal::config('system.maintenance')->getOriginal('message', FALSE);

To get the override values just use:

method 1

$message = \Drupal::config('system.maintenance')->get('message');

method 2

$message = \Drupal::configFactory()->getEditable('system.maintenance')->get('message');