variableconfig.php

<?php
/**
 * @author Gerard van Helden <gerard@zicht.nl>
 * @copyright Zicht Online <http://zicht.nl>
 */
 
require_once 'vendor/autoload.php';

class MyConf implements \Symfony\Component\Config\Definition\ConfigurationInterface
{
    /**
     * Generates the configuration tree builder.
     *
     * @return \Symfony\Component\Config\Definition\Builder\TreeBuilder The tree builder
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new \Symfony\Component\Config\Definition\Builder\TreeBuilder();

        $root = $treeBuilder->root('task');

        $root
            ->children()
                ->arrayNode('tasks')
                    ->prototype('array')
                        ->beforeNormalization()->always(function($v) {
                            $params = $v;
                            unset($params['type']);
                            return array(
                                'type' => $v['type'],
                                $v['type'] => $params
                            );
                        })->end()
                        ->children()
                            ->scalarNode('type')->end()
                            ->arrayNode('a')
                                ->children()
                                    ->variableNode('param_valid_for_a')->end()
                                ->end()
                            ->end()
                            ->arrayNode('b')
                                ->children()
                                    ->variableNode('param_valid_for_b')->end()
                                ->end()
                            ->end()
                        ->end()
                    ->end()
                ->end()
            ->end()
        ;

        return $treeBuilder;
    }
}

$yml = <<<EOYML
tasks:
    -
        type: "a"
        param_valid_for_a: "value1"
    -
        type: "a"
        param_valid_for_a: "value2"
#        param_valid_for_b: "value2"
    -
        type: "b"
        param_valid_for_b: "value"
EOYML;

$processor = new \Symfony\Component\Config\Definition\Processor();
$configs = array();
$configs[]= \Symfony\Component\Yaml\Yaml::parse($yml);

$result = $processor->processConfiguration(new MyConf(), $configs);
print_r($result);