How to handle global variables from legacy codes in Symfony?

In Sympony, Use dependency injection to avoid global entirely.

1、Replace global $config with Symfony Parameters​

# config/services.yaml
parameters:
    db_host: '%env(DB_HOST)%'  # Load from .env
    app.some_value: 'foo'      # Hardcoded value

​​Migrating Global Variables to Symfony (Best Practices)​​
When moving from a legacy PHP app with ​​global variables​​ to Symfony, you should ​​avoid globals entirely​​ and use Symfony’s dependency injection (DI) and configuration systems instead. Here’s how to handle common cases:

​​1. Replace global $configwith Symfony Parameters​​
​​Legacy Code (❌ Bad)​​
global $config;
$dbHost = $config['db_host'];
​​Symfony Way (✅ Good)​​
​​Step 1: Define Parameters​​
# config/services.yaml
parameters:
    db_host: '%env(DB_HOST)%'  # Load from .env
    app.some_value: 'foo'      # Hardcoded value
​​Step 2: Inject Parameters​​
use Symfony\Component\DependencyInjection\ParameterBag\ParameterBagInterface;

class DatabaseService
{
    public function __construct(
        private ParameterBagInterface $params
    ) {}

    public function connect()
    {
        $dbHost = $this->params->get('db_host');
    }
}

2、Replace global $db with a service

class DatabaseService
{
    private \PDO $connection;

    public function __construct(string $dsn)
    {
        $this->connection = new \PDO($dsn);
    }
}

// Register in services.yaml:
services:
    App\Service\DatabaseService:
        arguments:
            $dsn: 'mysql:host=%db_host%;dbname=%db_name%'

3、Replace global $user with Symfony Security​

use Symfony\Bundle\SecurityBundle\Security;

class SomeService
{
    public function __construct(
        private Security $security
    ) {}

    public function doSomething()
    {
        $user = $this->security->getUser(); // Typed to your User entity
        $userId = $user->getId();
    }
}

4、Replace global $loggerwith Monolog​

use Psr\Log\LoggerInterface;

class PaymentService
{
    public function __construct(
        private LoggerInterface $logger
    ) {}

    public function process()
    {
        $this->logger->info('Payment processed');
    }
}

5、Replace define('CONSTANT', value)with Parameters or Enums​

# config/services.yaml
parameters:
    app.debug: '%env(bool:APP_DEBUG)%'

enum AppSettings: bool
{
    case DEBUG = true;
}

if (AppSettings::DEBUG->value) { ... }

6、Replace Superglobals ($_SESSION, $_POST) with Symfony Request​

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Session\SessionInterface;

class FormController
{
    public function submit(
        Request $request,
        SessionInterface $session
    ) {
        $name = $request->request->get('name'); // $_POST
        $session->set('user_id', 123); // $_SESSION
    }
}

7、Replace require_once 'functions.php'with Services​

class LegacyFunctionsService
{
    public function doSomething()
    {
        // Replaces some_legacy_function()
    }
}

class ModernService
{
    public function __construct(
        private LegacyFunctionsService $legacy
    ) {}

    public function process()
    {
        $this->legacy->doSomething();
    }
}