Dependency Injection in PHP (Simple and Practical)

Dependency Injection (DI) means: instead of creating dependencies inside a class, you pass them in.

This makes code:

• easier to test
• easier to change later
• more professional

Bad (hard to test)

class OrdersService {
  public function createOrder() {
    $pdo = new PDO("...");
    // hard to test because DB is inside
  }
}

Good (inject dependency)

class OrdersService {
  public function __construct(private PDO $pdo) {}

  public function createOrder() {
    // use $this->pdo
  }
}

Why this is better

flowchart LR
  A[Controller] --> B[OrdersService]
  B --> C[(PDO DB)]

Now in tests you can inject a fake PDO or a mock wrapper.

Next: PHP exceptions, custom exceptions, and clean error responses.