KnpMarkdownBundle/DependencyInjection/Configuration.php

<?php

namespace Knp\Bundle\MarkdownBundle\DependencyInjection;

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;

class Configuration implements ConfigurationInterface
{
    /**
     * Generates the configuration tree builder.
     *
     * @return TreeBuilder
     */
    public function getConfigTreeBuilder(): TreeBuilder
    {
        $treeBuilder = new TreeBuilder('knp_markdown');
        $rootNode = $treeBuilder->getRootNode();

        $rootNode
            ->addDefaultsIfNotSet()
            ->children()
                ->arrayNode('parser')
                    ->addDefaultsIfNotSet()
                    ->children()
                        ->scalarNode('service')->cannotBeEmpty()->defaultValue('markdown.parser.max')->end()
                    ->end()
                ->end()
            ->end()
        ->end();

        return $treeBuilder;
    }
}