What is dependency injection in Laravel?

 Difficulty: Intermediate → Advanced

Dependency injection means providing a class with the objects it needs instead of having the class construct those objects itself.

Consider:

class OrderController
{
    public function __construct(
        private OrderService $orderService
    ) {}
}

Laravel's service container can resolve the dependency.

The controller doesn't need:

$this->orderService = new OrderService();

This provides several benefits:

Loose coupling
Testability
Maintainability
Replaceable implementations
Cleaner architecture

For example, you could depend on an interface:

interface PaymentGateway
{
    public function charge(int $amount): bool;
}

and inject an implementation.

This becomes especially useful when an application supports:

Stripe
PayPal
Razorpay
Internal payment gateway
Mock payment gateway for testing