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What is a service container, and what problem does it solve when an application has many interdependent classes? How does autowiring work in a service container, and how does it use type hints to automatically resolve a class's dependencies? How do you bind an interface to a specific concrete implementation within a service container, and why is this a common and valuable pattern? What is the difference between singleton and transient bindings in a service container, and when should each be used? How does the dependency inversion principle relate to dependency injection, and how does designing around abstractions rather than concrete classes improve long term maintainability? How can overuse of a service container, sometimes called the service locator anti pattern, actually harm code quality despite the container itself being a useful tool?
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What is dependency injection, and how does it improve the design of PHP applications compared to creating dependencies directly inside a class?

Beginner

Answer

Dependency injection is a design pattern where a class receives the objects it depends on from an outside source, typically through its constructor, rather than creating those dependencies itself internally, which makes the class more flexible, easier to test since dependencies can be replaced with test doubles, and more loosely coupled since the class does not need to know exactly how its dependencies are constructed.
class OrderService {
    private $mailer;
    public function __construct(Mailer $mailer) {
        $this->mailer = $mailer;
    }
}
$service = new OrderService(new Mailer());
Real-world example An order processing class receives a Mailer object through its constructor rather than creating one internally, allowing tests to easily substitute a fake mailer that simply records what would have been sent, without actually sending real emails during testing.

Common follow-ups: What is the difference between constructor injection and setter injection?;Why does dependency injection make unit testing significantly easier?

OOP;Unit Testing with PHPUnit
All 7 Dependency Injection & Service Containers questions