Dependency Injection

Also Known As

TL;DR

Explanation

Dependency injection (DI) means a class receives its collaborators (database connection, mailer, logger) as constructor arguments or method parameters rather than instantiating them internally with new. This makes the class testable (you can pass mock collaborators in tests), configurable (swap implementations without changing the class), and loosely coupled (the class depends on an interface, not a concrete implementation). Constructor injection is the preferred form — it makes dependencies explicit and the class impossible to create in an invalid state.

Diagram

flowchart LR
    subgraph Without_DI
        CLASS2[UserService creates its own<br/>new DatabaseConnection<br/>new EmailClient]
        TIGHT[Tightly coupled<br/>untestable<br/>hardcoded deps]
    end
    subgraph With_DI
        CONSTR[Constructor injection<br/>deps passed in]
        IFACE[Depend on interface<br/>not concrete class]
        CONSTR & IFACE --> LOOSE[Loosely coupled<br/>testable<br/>swappable]
    end
    subgraph Container
        IOC[IoC Container<br/>auto-resolves dependencies]
        BIND[bind interface to implementation]
        IOC --> BIND
    end
style TIGHT fill:#f85149,color:#fff
style LOOSE fill:#238636,color:#fff
style IOC fill:#6e40c9,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Injecting the container itself (service locator anti-pattern) — inject specific dependencies instead.
• Using constructor injection for optional dependencies — use setter injection or nullable defaults for those.
• Newing up dependencies inside methods (new Mailer()) instead of receiving them from outside.
• Registering everything as a singleton when it should be transient, causing state leakage between requests.

Code Examples

class OrderService {
    public function place(Cart $cart): Order {
        $mailer = new Mailer();       // hard-coded dependency
        $db     = new Database();     // impossible to swap/mock
        $db->save($cart);
        $mailer->send($cart->user, 'Your order is placed');
    }
}

class OrderService {
    public function __construct(private MailerInterface $mailer) {}

    public function notify($order) {
        $this->mailer->send($order->email, 'Order confirmed');
    }
}

References

• ↗ https://en.wikipedia.org/wiki/Dependency_injection
• ↗ https://phptherightway.com/#dependency_injection

Tags

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