nexmo-bundle/DependencyInjection/Configuration.php

<?php

namespace Jhg\NexmoBundle\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/extension.html#cookbook-bundles-extension-config-class}
 */
class Configuration implements ConfigurationInterface
{
    /**
     * {@inheritDoc}
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('jhg_nexmo');

        $rootNode
        	->children()
	        	->scalarNode('api_key')
                    ->isRequired()
                ->end()

	        	->scalarNode('api_secret')
                    ->isRequired()
                ->end()

	        	->scalarNode('from_name')
                    ->validate()
                        ->ifTrue(function ($s) {
                            return (strlen($s)>11 || strlen($s)<2) && preg_match('/^[0-9a-z]{11}$/i', $s) !== 1;
                        })
                            ->thenInvalid('Invalid from_name, only alphanumeric characters are allowed')
                        ->end()
                ->end()

	        	->booleanNode('disable_delivery')
                    ->defaultFalse()
                ->end()
        	->end()
        ;
        
        return $treeBuilder;
    }
}