I will actually work on remove the when() method. It is not really needed in PHP, right?

I like it in yaml and XML because it allows me to write simple logic, however, in PHP I do not need a feature to "help me with simple logic".

I was thinking about this config based on your example:

use Symfony\Config\FrameworkConfig;

return static function (FrameworkConfig $framework)
{
    $framework->when('test')
        ->test(true)
        ->session()->storageFactoryId('session.storage.mock_file')
    ;
};

Not sure if possible though (I didn't check if the generated Config is fluent).

Yes, That is the alternative way of doing things. The implementation is more difficult and it is less obvious what is happening.

Hm.. So we are loading callbacks on 4 places.

1. DI: Microkernel trait
2. DI: PhpFileLoader
3. Router: Microkernel trait
4. Router: PhpFileLoader

For 2, we have a fancy resolver of the parameters. So order does not matter, but parameter name does. For the others, order does matter but not parameter name.

The fancy resolver is needed for the ConfigBuilders. I would like to use fancy resolving for all of these, but either I write (almost) the same code on 3 more places or I try to do something complex...

So, instead of doing fancy argument resolving on all places. I added ContainerConfigurator::env(): ?string and RoutingConfigurator::env(): ?string.

I've updated the description. This PR is ready for a review.

Thank you @Nyholm.

Woho. Let's start write php config