How does DIP improve testability?

DIP improves testability by making it possible to substitute real dependencies with test doubles (mocks, stubs, fakes). When a class depends on a concrete implementation (e.g., MySQLDatabase), unit testing it requires a real database — slow, fragile, and stateful. When it depends on an interface (DatabaseInterface), tests can inject a MockDatabase that returns controlled data without network calls. This makes tests fast (no I/O), isolated (no shared state), deterministic (predictable responses), and focused (testing only the class's logic). DIP combined with constructor injection enables new OrderService(new MockDatabase()) in test setup. This is why frameworks like Laravel use DIP extensively — the service container makes both production and test wiring seamless.