DI

Dependency Injection (DI) is a design pattern where an object receives its dependencies from the outside instead of creating them itself. It is a form of Inversion of Control: the responsibility for creating and configuring services is handled by a container or provider (for example Laravel's service container), not by the consumer. In PHP backend/web development DI reduces coupling, improves testability, and makes it simple to swap or configure implementations (for example different repositories or gateways).

Imagine you have a PaymentGateway interface in a Laravel app and multiple implementations (Stripe, PayPal). In a ServiceProvider you bind the interface to a concrete class, e.g. $this->app->bind(PaymentGateway::class, StripePayment::class). Then inject the interface into a controller via the constructor: public function __construct(PaymentGateway $gateway) { $this->gateway = $gateway; }. For testing you can override that binding with a mock or fake so the controller can be tested without performing real payments. This avoids using new inside the controller and keeps the code modular and easily replaceable.