DependencyInjection/Configuration.php
public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('abc_scheduler');
        $supportedDrivers = array('orm', 'custom');

        $rootNode
            ->children()
                ->scalarNode('db_driver')
                    ->cannotBeOverwritten()
                    ->defaultValue('orm')
                    ->validate()
                        ->ifNotInArray($supportedDrivers)
                        ->thenInvalid('The driver %s is not supported. Please choose one of '.json_encode($supportedDrivers))
                    ->end()
                ->end()
                ->scalarNode('model_manager_name')
                    ->defaultNull()
                ->end()
            ->end();

        return $treeBuilder;
    }
public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('abc_scheduler');
        $supportedDrivers = array('orm', 'custom');

        $rootNode
            ->children()
                ->scalarNode('db_driver')
                    ->cannotBeOverwritten()
                    ->defaultValue('orm')
                    ->validate()
                        ->ifNotInArray($supportedDrivers)
                        ->thenInvalid('The driver %s is not supported. Please choose one of '.json_encode($supportedDrivers))
                    ->end()
                ->end()
                ->scalarNode('model_manager_name')
                    ->defaultNull()
                ->end()
            ->end();

        return $treeBuilder;
    }
{@inheritDoc}