config.yml

services:
    dumb:
        class: Acme\DumbService
        arguments:
            - "name"
            - "hello": { "world": { "foo": %mailer_host% } }

DemoController.php

<?php

...
        $dumb_service = $this->container->get('dumb');
        $dumb_service->dump();
...

DumbService.php

<?php

namespace Acme;

class DumbService
{

        public function __construct($name, $options)
        {
                $this->name = $name;
                $this->options = $options;
        }

        public function dump()
        {
                var_dump($this->name, $this->options);
        }

}

output.txt

string 'name' (length=4)
array
  'hello' => 
    array
      'world' => 
        array
          'foo' => string 'localhost' (length=9)