Installation

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$ composer require symfony/dependency-injection

Basic Usage

You might have a class like the following Mailer that you want to make available as a service:

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class Mailer
{
    private $transport;

    public function __construct()
    {
        $this->transport = 'sendmail';
    }

    // ...
}

You can register this in the container as a service:

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use Symfony\Component\DependencyInjection\ContainerBuilder;

$containerBuilder = new ContainerBuilder();
$containerBuilder->register('mailer', 'Mailer');

An improvement to the class to make it more flexible would be to allow the container to set the transport used. If you change the class so this is passed into the constructor:

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class Mailer
{
    private $transport;

    public function __construct($transport)
    {
        $this->transport = $transport;
    }

    // ...
}

Then you can set the choice of transport in the container:

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use Symfony\Component\DependencyInjection\ContainerBuilder;

$containerBuilder = new ContainerBuilder();
$containerBuilder
    ->register('mailer', 'Mailer')
    ->addArgument('sendmail');

This class is now much more flexible as you have separated the choice of transport out of the implementation and into the container.

Which mail transport you have chosen may be something other services need to know about. You can avoid having to change it in multiple places by making it a parameter in the container and then referring to this parameter for the Mailer service's constructor argument:

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use Symfony\Component\DependencyInjection\ContainerBuilder;

$containerBuilder = new ContainerBuilder();
$containerBuilder->setParameter('mailer.transport', 'sendmail');
$containerBuilder
    ->register('mailer', 'Mailer')
    ->addArgument('%mailer.transport%');

Now that the mailer service is in the container you can inject it as a dependency of other classes. If you have a NewsletterManager class like this:

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class NewsletterManager
{
    private $mailer;

    public function __construct(\Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    // ...
}

When defining the newsletter_manager service, the mailer service does not exist yet. Use the Reference class to tell the container to inject the mailer service when it initializes the newsletter manager:

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use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;

$containerBuilder = new ContainerBuilder();

$containerBuilder->setParameter('mailer.transport', 'sendmail');
$containerBuilder
    ->register('mailer', 'Mailer')
    ->addArgument('%mailer.transport%');

$containerBuilder
    ->register('newsletter_manager', 'NewsletterManager')
    ->addArgument(new Reference('mailer'));

If the NewsletterManager did not require the Mailer and injecting it was only optional then you could use setter injection instead:

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class NewsletterManager
{
    private $mailer;

    public function setMailer(\Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    // ...
}

You can now choose not to inject a Mailer into the NewsletterManager. If you do want to though then the container can call the setter method:

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use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;

$containerBuilder = new ContainerBuilder();

$containerBuilder->setParameter('mailer.transport', 'sendmail');
$containerBuilder
    ->register('mailer', 'Mailer')
    ->addArgument('%mailer.transport%');

$containerBuilder
    ->register('newsletter_manager', 'NewsletterManager')
    ->addMethodCall('setMailer', [new Reference('mailer')]);

You could then get your newsletter_manager service from the container like this:

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use Symfony\Component\DependencyInjection\ContainerBuilder;

$containerBuilder = new ContainerBuilder();

// ...

$newsletterManager = $containerBuilder->get('newsletter_manager');

Avoiding your Code Becoming Dependent on the Container

Whilst you can retrieve services from the container directly it is best to minimize this. For example, in the NewsletterManager you injected the mailer service in rather than asking for it from the container. You could have injected the container in and retrieved the mailer service from it but it would then be tied to this particular container making it difficult to reuse the class elsewhere.

You will need to get a service from the container at some point but this should be as few times as possible at the entry point to your application.

Setting up the Container with Configuration Files

As well as setting up the services using PHP as above you can also use configuration files. This allows you to use XML or YAML to write the definitions for the services rather than using PHP to define the services as in the above examples. In anything but the smallest applications it makes sense to organize the service definitions by moving them into one or more configuration files. To do this you also need to install the Config component.

Loading an XML config file:

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

$containerBuilder = new ContainerBuilder();
$loader = new XmlFileLoader($containerBuilder, new FileLocator(__DIR__));
$loader->load('services.xml');

Loading a YAML config file:

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use Symfony\Component\Config\FileLocator;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;

$containerBuilder = new ContainerBuilder();
$loader = new YamlFileLoader($containerBuilder, new FileLocator(__DIR__));
$loader->load('services.yaml');

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// ...
$loader->load('services.config', 'xml');

If you do want to use PHP to create the services then you can move this into a separate config file and load it in a similar way:

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use Symfony\Component\Config\FileLocator;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\PhpFileLoader;

$containerBuilder = new ContainerBuilder();
$loader = new PhpFileLoader($containerBuilder, new FileLocator(__DIR__));
$loader->load('services.php');

You can now set up the newsletter_manager and mailer services using config files:

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parameters:
    # ...
    mailer.transport: sendmail

services:
    mailer:
        class:     Mailer
        arguments: ['%mailer.transport%']
    newsletter_manager:
        class:     NewsletterManager
        calls:
            - [setMailer, ['@mailer']]

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<?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"
>
    <parameters>
        <!-- ... -->
        <parameter key="mailer.transport">sendmail</parameter>
    </parameters>

    <services>
        <service id="mailer" class="Mailer">
            <argument>%mailer.transport%</argument>
        </service>

        <service id="newsletter_manager" class="NewsletterManager">
            <call method="setMailer">
                <argument type="service" id="mailer"/>
            </call>
        </service>
    </services>
</container>

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namespace Symfony\Component\DependencyInjection\Loader\Configurator;

return static function (ContainerConfigurator $containerConfigurator) {
    $containerConfigurator->parameters()
        // ...
        ->set('mailer.transport', 'sendmail')
    ;

    $services = $containerConfigurator->services();
    $services->set('mailer', 'Mailer')
        ->args(['%mailer.transport%'])
    ;

    $services->set('mailer', 'Mailer')
        ->args([param('mailer.transport')])
    ;

    $services->set('newsletter_manager', 'NewsletterManager')
        ->call('setMailer', [service('mailer')])
    ;
};

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