Since I think Symfony 3.4, we can do the following to make a default value for an env var if it's not set : ```yaml parameters: env(SECRET): foo ``` It is mentionned all throughout the docs (example [here](https://symfony.com/blog/new-in-symfony-3-4-advanced-environment-variables), [here](https://symfony.com/doc/current/configuration/env_var_processors.html) and [here](https://symfony.com/doc/current/configuration.html#configuration-based-on-environment-variables)), but it is never detailed what it is and what's the point of using it. Maybe adding a section or a short explanation of what it is could be beneficial before using it as it is currently used ? I'm afraid I lack the writing skills to make a proper explanation on that to contribute efficiently on that issue. :/