dependency-injection

name	dependency-injection
description	Use this skill when wiring dependencies, configuring a DI container, designing plugin/extension points, or migrating away from legacy service locators in PHP. Triggers on questions about constructor injection, interface registration, swapping implementations, or testing without booting the container.

Dependency Injection Best Practices for PHP

When to Use a DI Container (and When NOT to)

A DI container is an infrastructure concern. Use it for applications that need flexible wiring, interface swapping, or framework integration. Libraries should use constructor injection directly and let the consumer resolve dependencies.

Use a DI Container For	Do Not Use a Container For
Applications with many interchangeable services	Libraries and packages
Need to swap implementations (test vs production)	Simple scripts with one or two classes
Framework integration (Symfony, Laravel)	Throwaway code or prototypes

The container should wire the application together at the edge, while the core remains ignorant of the container. This skill provides the rules for registering dependencies, migrating legacy locators, and injecting objects.

A Plugin is the purpose this wiring serves: define extension points (Ports) so new behavior is added by plugging in implementations (Adapters) at the composition root, without modifying the core. A plugin must extend the existing abstractions, never bypass them. It protects the framework / extension boundary — one of a family of boundary-protection patterns (with Gateway, Mapper, Remote Facade, DTO, and Special Case) that form the foundation of Hexagonal architecture; see boundary-protection-patterns.md.

Principles Behind the Wiring

• Dependency Inversion (DI). High-level modules depend on abstractions, not on low-level details. The application defines ports (interfaces); infrastructure supplies the adapters. This is the principle that makes a DI container useful and is the backbone of Hexagonal / Ports-and-Adapters architecture. See solid-principles.md.
• Narrow ports (Interface Segregation). Each port should expose only the behavior its clients actually use. A port with methods no caller needs forces adapters to implement dead code and couples clients to irrelevant surface area. Keep ports role-specific.
• Interface segregation vs. premature abstraction. Create an interface when there is a real second implementation, a boundary to invert, or a decorator to add. Do not introduce an interface for a single concrete class just "for flexibility" (or even just for testing, because mocks and stubs are usually fine) — that is premature abstraction (YAGNI). The workflow below encodes this check.

Numbered Workflows

1. Registering Dependencies

If deciding whether to create an interface and register it in the DI container:

1. Check for Multiple Implementations. Are there multiple implementations (e.g., StripeAdapter and PayPalAdapter)?
2. Check for Boundary Inversion. Is the application defining a port that the infrastructure implements?
3. Check for Decorators. Do you need to wrap the class with caching or logging?
4. If Yes to any: Create an interface and register the binding in the container configuration.
5. If No to all: Depend on the concrete class directly and let the container autowire it. Do not blindly create interfaces for every class — one interface with exactly one implementation and no second on the roadmap is dead weight.

2. Testing with Constructor Injection

If writing tests for classes that use constructor injection:

1. For Domain/Handler Tests: Construct the object using new. Pass in test doubles (mocks or fakes) for any infrastructure dependencies. Do not boot the DI container.
2. For Integration/Feature Tests: Ask the test framework to resolve the controller or handler from the DI container.

3. Migrating Legacy Code

If modifying a legacy class that uses a service locator (Container::getInstance()->get()):

1. Follow the Migration Sequence. Read the Migration Sequence for Legacy Containers.
2. Extract to Constructor. Find every internal getInstance() call and add those dependencies as typed constructor properties.
3. Fix Callers. Update all instantiations of the legacy class to pass the dependencies.
4. Use Shims if Necessary. If the class is instantiated in 50 places you cannot change, use the temporary shim pattern to bridge the gap.

Boundaries

Always Do

• Always use constructor injection for services, handlers, repositories, adapters, and listeners.
• Always keep the container wiring logic at the framework edge (composition roots, service providers).
• Always keep the domain core free of framework dependencies. It should resolve from tests or a composition root with no framework boot.
• Always treat a plugin as a first-class architectural citizen: it implements a defined Port and obeys the same dependency rule and invariants as core code (see plugin.md).

Ask First

• Ask before introducing a legacy shim if it is feasible to simply update all callers instead.
• Ask before adding an abstraction that a concrete class would serve just as well. Reach for an interface only when a second implementation, a boundary to invert, or a decorator genuinely exists — not "in case we need it later."
• Ask before wrapping a single dependency in a one-method delegating class; prefer passing the dependency directly or as a named function.

Never Do

• Never use service locators inside domain objects, application handlers, or policies. They hide dependencies and couple core code to the framework.
• Never let a plugin become a backdoor: it must not reach into core internals, write to the database directly, or skip domain invariants. Plug in via the defined Port only.
• Never add a new dependency when an existing one already does the job; reuse before reaching for a package.

Related Patterns

• The Factory pattern (variant selection, composition-root wiring, Singleton as anti-pattern) is covered in creation-patterns.md.
• The five SOLID principles and the clean-code foundations that motivate this wiring are in solid-principles.md and clean-code-foundations.md.