Dependency Injection & Containers

Rather than a class constructing its own dependencies, they're provided ("injected") from outside — usually through the constructor.

interface EmailSender
{
    public function send(string $to, string $message): void;
}

class OrderService
{
    public function __construct(private EmailSender $emailSender) {}

    public function placeOrder(string $customerEmail): void
    {
        $this->emailSender->send($customerEmail, 'Order confirmed!');
    }
}

A dependency injection container (built into Laravel and Symfony) automatically resolves and constructs these dependencies based on type hints, so you rarely wire things up by hand in a framework-based project — but the underlying idea is exactly the constructor injection shown above.