Configuration.php

<?php

namespace Welp\IcalBundle\DependencyInjection;

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

/**
 * This is the class that validates and merges configuration from your app/config files.
 *
 * To learn more see {@link http://symfony.com/doc/current/cookbook/bundles/configuration.html}
 */
class Configuration implements ConfigurationInterface
{
    public function getConfigTreeBuilder(): TreeBuilder
    {
        $treeBuilder = new TreeBuilder('welp_ical');
        $rootNode = $treeBuilder->getRootNode();

        // Here you should define the parameters that are allowed to
        // configure your bundle. See the documentation linked above for
        // more information on that topic.

        $rootNode->children()
            ->scalarNode('default_prodid')->defaultValue('-//WelpIcalBundle//Calendar App//FR')->end()
            ->scalarNode('default_timezone')->defaultNull()->end();

        return $treeBuilder;
    }
}