extra_config_yaml.php

<?php

/* Configure parameters and service in config.yml:
 *
 *     imports:
 *         - { resource: parameters.extras.yml }
 *
 *     services:
 *         dynamic.parameters:
 *             class: Ctors\MyBundle\Util\DynamicParameters
 *             arguments: [ '%kernel.root_dir%', 'parameters.extras.yml' ]
 *
 * Use a parameter (eg: in Controller):
 *
 *     $this->container->getParameter('something.active');
 * 
 * Change a parameter (eg: from Controller):
 *     
 *     $dynamicParameters = $this->container->get('dynamic.parameters');
 *     $parameters = $dynamicParameters->getParameters();
 *     $parameters['parameters']['something.active'] = 'Off';
 *     $dynamicParameters->saveParameters($parameters);
 *
 * Just use '%something.active%' in config.yml for configuring other services.
 */

namespace Ctors\MyBundle\Util;

use Symfony\Component\Yaml\Dumper,
    Symfony\Component\Yaml\Parser;

class DynamicParameters
{
    private $file;
    private $parameters;

    public function __construct($rootDir, $filename)
    {
        $this->file = "{$rootDir}/config/{$filename}";
        $this->ensureDefaults();
    }

    private function ensureDefaults()
    {
        if (file_exists($this->file) === false) {
            $this->saveParameters(
                array(
                    'parameters' => array(
                        'something.active' => 'On'
                    )
                )
            );
        }
    }

    /**
     * @return array
     */
    public function getParameters()
    {
        $parser = new Parser();
        $yaml = file_get_contents($this->file);

        return $parser->parse($yaml);
    }

    /**
     * @param array $parameters
     */
    public function saveParameters($parameters)
    {
        $dumper = new Dumper();
        $yaml = $dumper->dump($parameters);
        file_put_contents($this->file, $yaml);
    }
}