gistfile1.txt

     private function addHttpClientOptionsSection(NodeBuilder $rootNode): NodeBuilder
     {
        return $rootNode
            ->integerNode('max_host_connections')
                ->info('The maximum number of connections to a single host.')
                ->defaultValue(6)
            ->end()
            ->arrayNode('default_options')
                ->children()
                    ->fixXmlConfig('header')
                    // ...
                    ->arrayNode('headers')
                        ->info('Associative array: header => value(s).')
                        ->useAttributeAsKey('name')
                        ->normalizeKeys(false)
                        ->variablePrototype()->end()
                    ->end()
                    // ...
                ->end()
            ->end()
        ;
    }