Load PHP class from custom module within settings.php file #php #drupal8+

custom.settings.php

<?php

/**
 * @file
 * Load PHP class from custom module within settings.php file.
 */

if (class_exists(ClassLoader::class)) {
  $class_loader = new ClassLoader();
  $class_loader->addPsr4('Drupal\\mymodule\\', DRUPAL_ROOT . '/modules/custom/mymodule/src');
  $class_loader->register();
}

if (!class_exists('Drupal\mymodule\MyModuleConstants')) {
  return;
}

$some_constants = \Drupal\mymodule\MymoduleConstants::LEGACY_DB_KEY;