AppKernel.php

class AppKernel extends Kernel
{
  
  ...
  
  /**
   * Add loader to load settings from /etc/bigproject/settings.conf
   */
  protected function getContainerLoader(ContainerInterface $container)
  {
    $cl = parent::getContainerLoader($container);

    // Add additional loader to the resolver
    $resolver = $cl->getResolver();
    $resolver->addLoader(new BigProject\MainBundle\ConfFileLoader($container, new FileLocator(array())));

    return $cl;
  }
  
}

ConfFileLoader.php

<?php

namespace BigProject\MainBundle;

use Symfony\Component\Config\Resource\FileResource;
use Symfony\Component\DependencyInjection\Exception\InvalidArgumentException;
use Symfony\Component\DependencyInjection\Loader\FileLoader;

class ConfFileLoader extends FileLoader
{
    /**
     * Loads a resource.
     *
     * @param mixed  $file The resource
     * @param string $type The resource type
     *
     * @throws InvalidArgumentException When ini file is not valid
     */
    public function load($file, $type = null)
    {
        $path = $this->locator->locate($file);

        $this->container->addResource(new FileResource($path));

        $result = parse_ini_file($path, true);
        if (false === $result || array() === $result) {
            throw new InvalidArgumentException(sprintf('The "%s" file is not valid.', $file));
        }

        foreach ($result as $key => $section) {
            foreach ($section as $name => $value) {
                $this->container->setParameter($key.".".$name, $value);
            }
        }
    }

    /**
     * Returns true if this class supports the given resource.
     *
     * @param mixed  $resource A resource
     * @param string $type     The resource type
     *
     * @return Boolean true if this class supports the given resource, false otherwise
     */
    public function supports($resource, $type = null)
    {
        return is_string($resource) && 'conf' === pathinfo($resource, PATHINFO_EXTENSION);
    }
}

config.yml

imports:
  - { resource: parameters.yml }

....

markup_solarium:
    clients:
        default:
            host: %solr.host%
            port: %solr.port%
            path: %solr.path%
            core: %solr.core%
            timeout: 5

mainconfig.conf

[solr]
  host = 10.10.1.10
  port = 8080
  path = /solr
  core = mycore

parameters.yml

parameters:
  solr.host: 10.10.1.10
  solr.port: 8080
  solr.path: /solr
  solr.core: mycore