Processing the $configs Array

First things first, you have to create an extension class as explained in How to Load Service Configuration inside a Bundle.

Whenever a user includes the acme_social key (which is the DI alias) in a configuration file, the configuration under it is added to an array of configurations and passed to the load() method of your extension (Symfony automatically converts XML and YAML to an array).

For the configuration example in the previous section, the array passed to your load() method will look like this:

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[
    [
        'twitter' => [
            'client_id' => 123,
            'client_secret' => 'your_secret',
        ],
    ],
]

Notice that this is an array of arrays, not just a single flat array of the configuration values. This is intentional, as it allows Symfony to parse several configuration resources. For example, if acme_social appears in another configuration file - say config/packages/dev/acme_social.yaml - with different values beneath it, the incoming array might look like this:

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[
    // values from config/packages/acme_social.yaml
    [
        'twitter' => [
            'client_id' => 123,
            'client_secret' => 'your_secret',
        ],
    ],
    // values from config/packages/dev/acme_social.yaml
    [
        'twitter' => [
            'client_id' => 456,
        ],
    ],
]

The order of the two arrays depends on which one is set first.

But don't worry! Symfony's Config component will help you merge these values, provide defaults and give the user validation errors on bad configuration. Here's how it works. Create a Configuration class in the DependencyInjection directory and build a tree that defines the structure of your bundle's configuration.

The Configuration class to handle the sample configuration looks like:

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// src/Acme/SocialBundle/DependencyInjection/Configuration.php
namespace Acme\SocialBundle\DependencyInjection;

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

class Configuration implements ConfigurationInterface
{
    public function getConfigTreeBuilder(): TreeBuilder
    {
        $treeBuilder = new TreeBuilder('acme_social');

        $treeBuilder->getRootNode()
            ->children()
                ->arrayNode('twitter')
                    ->children()
                        ->integerNode('client_id')->end()
                        ->scalarNode('client_secret')->end()
                    ->end()
                ->end() // twitter
            ->end()
        ;

        return $treeBuilder;
    }
}

This class can now be used in your load() method to merge configurations and force validation (e.g. if an additional option was passed, an exception will be thrown):

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// src/Acme/SocialBundle/DependencyInjection/AcmeSocialExtension.php
public function load(array $configs, ContainerBuilder $container): void
{
    $configuration = new Configuration();

    $config = $this->processConfiguration($configuration, $configs);

    // you now have these 2 config keys
    // $config['twitter']['client_id'] and $config['twitter']['client_secret']
}

The processConfiguration() method uses the configuration tree you've defined in the Configuration class to validate, normalize and merge all the configuration arrays together.

Now, you can use the $config variable to modify a service provided by your bundle. For example, imagine your bundle has the following example config:

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<!-- src/Acme/SocialBundle/Resources/config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd"
>
    <services>
        <service id="acme.social.twitter_client" class="Acme\SocialBundle\TwitterClient">
            <argument></argument> <!-- will be filled in with client_id dynamically -->
            <argument></argument> <!-- will be filled in with client_secret dynamically -->
        </service>
    </services>
</container>

In your extension, you can load this and dynamically set its arguments:

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// src/Acme/SocialBundle/DependencyInjection/AcmeSocialExtension.php
// ...

use Symfony\Component\Config\FileLocator;
use Symfony\Component\DependencyInjection\Loader\XmlFileLoader;

public function load(array $configs, ContainerBuilder $container): void
{
    $loader = new XmlFileLoader($container, new FileLocator(dirname(__DIR__).'/Resources/config'));
    $loader->load('services.xml');

    $configuration = new Configuration();
    $config = $this->processConfiguration($configuration, $configs);

    $definition = $container->getDefinition('acme.social.twitter_client');
    $definition->replaceArgument(0, $config['twitter']['client_id']);
    $definition->replaceArgument(1, $config['twitter']['client_secret']);
}

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// src/Acme/HelloBundle/DependencyInjection/AcmeHelloExtension.php
namespace Acme\HelloBundle\DependencyInjection;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\DependencyInjection\ConfigurableExtension;

class AcmeHelloExtension extends ConfigurableExtension
{
    // note that this method is called loadInternal and not load
    protected function loadInternal(array $mergedConfig, ContainerBuilder $container): void
    {
        // ...
    }
}

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public function load(array $configs, ContainerBuilder $container): void
{
    $config = [];
    // let resources override the previous set value
    foreach ($configs as $subConfig) {
        $config = array_merge($config, $subConfig);
    }

    // ... now use the flat $config array
}

Make your Config Tree ready for XML

The Config component provides some methods by default to allow it to correctly process XML configuration. See "Defining and Processing Configuration Values" of the component documentation. However, you can do some optional things as well, this will improve the experience of using XML configuration:

Choosing an XML Namespace

In XML, the XML namespace is used to determine which elements belong to the configuration of a specific bundle. The namespace is returned from the Extension::getNamespace() method. By convention, the namespace is a URL (it doesn't have to be a valid URL nor does it need to exist). By default, the namespace for a bundle is http://example.org/schema/dic/DI_ALIAS, where DI_ALIAS is the DI alias of the extension. You might want to change this to a more professional URL:

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// src/Acme/HelloBundle/DependencyInjection/AcmeHelloExtension.php

// ...
class AcmeHelloExtension extends Extension
{
    // ...

    public function getNamespace(): string
    {
        return 'http://acme_company.com/schema/dic/hello';
    }
}

Providing an XML Schema

XML has a very useful feature called XML schema. This allows you to describe all possible elements and attributes and their values in an XML Schema Definition (an XSD file). This XSD file is used by IDEs for auto completion and it is used by the Config component to validate the elements.

In order to use the schema, the XML configuration file must provide an xsi:schemaLocation attribute pointing to the XSD file for a certain XML namespace. This location always starts with the XML namespace. This XML namespace is then replaced with the XSD validation base path returned from Extension::getXsdValidationBasePath() method. This namespace is then followed by the rest of the path from the base path to the file itself.

By convention, the XSD file lives in the Resources/config/schema/, but you can place it anywhere you like. You should return this path as the base path:

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// src/Acme/HelloBundle/DependencyInjection/AcmeHelloExtension.php

// ...
class AcmeHelloExtension extends Extension
{
    // ...

    public function getXsdValidationBasePath(): string
    {
        return __DIR__.'/../Resources/config/schema';
    }
}

Assuming the XSD file is called hello-1.0.xsd, the schema location will be https://acme_company.com/schema/dic/hello/hello-1.0.xsd:

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<!-- config/packages/acme_hello.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:acme-hello="http://acme_company.com/schema/dic/hello"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd
        http://acme_company.com/schema/dic/hello
        https://acme_company.com/schema/dic/hello/hello-1.0.xsd"
>
    <acme-hello:config>
        <!-- ... -->
    </acme-hello:config>

    <!-- ... -->
</container>