In this PR I want to discuss the following config format:

<?php

use Symfony\Component\DependencyInjection\ContainerInterface;

return [
    'parameters' => [
        'foo' => 42,
        'bar' => 'bar'
    ],

    'services' => [
        // à la Pimple, by default
        'foo' => [
            'class' => FooBarBaz::class,
            'factory_method' => function (ContainerInterface $container) {
                return new FooBarBaz($container->getParameter('foo'));
            }
        ],

        // with arguments
        'bar' => [
            'class' => FooBarBaz::class,
            'factory_method' => function ($bar) {
                return new FooBarBaz($bar);
            },
            'arguments' => ['%bar%']
        ]
    ]
];

There are some reasons to use plain arrays instead of YAML/XML in some cases:

1. Short array syntax was introduced years ago and it minimises difference between YAML and plain array in readability.
2. Class name resolution allows IDE to find class usages, apply renaming, go to class by click, etc.
3. Expression language is nice, but it looks awkward in YAML/XML configs (well, at least for me). My point is to use closures for inline logic instead.

The main disadvantage of plain array against YAML/XML is lack of sandboxing. It can lead abusing of code in configs. But on other hand pure PHP config format has the same problem.

Unfortunately, I see some problems in order to create real PR:

1. My PR depends on Super Closure library for dumping closure's code, but I'm not sure if you accept it. By the way, only context free closures can be dumped, it's some sort of restriction.
2. PhpFileLoader already holds *.php extension, so order of loaders matters.

What do you think?