SOLID principles help PHP developers reduce coupling, clarify responsibilities, and design code that can evolve safely.
SOLID is about change
SOLID is sometimes taught as a checklist, but its real purpose is to manage change. Every principle points at a common failure mode: classes doing too much, code that requires modification for every new case, subclasses that break expectations, interfaces that force unused methods, and high-level logic tied to low-level details. In a real project, these problems show up as slow development and risky releases.
Single Responsibility Principle
A class should have one primary reason to change. A controller that validates permissions, calculates prices, writes database rows, and sends email has many reasons. Splitting the workflow into an action, a policy, a calculator, a repository, and a notifier makes each piece easier to understand. The goal is not tiny classes for their own sake; the goal is change isolation.
Open Closed Principle
Code should be open for extension and closed for modification. In practice, that often means replacing a growing conditional with strategies, handlers, or a registry. If a shipping calculator changes every time a new carrier is added, the system is not closed to modification. A ShippingMethod interface lets new carriers be added as new classes while existing code remains stable.
Liskov Substitution Principle
A subtype should be usable wherever its parent type is expected. Violations happen when a child class throws unsupported exceptions, ignores required behavior, or changes the meaning of a method. This is one reason composition can be safer than inheritance. If subclasses cannot honor the same contract, they probably should not share that parent type.
Interface Segregation Principle
Clients should not be forced to depend on methods they do not use. A large ContentServiceInterface with publish, archive, render, translate, import, and export methods will pressure every implementation to fake behavior. Smaller interfaces keep dependencies honest. A renderer needs RenderableContent, while an importer needs ImportableContent.
Dependency Inversion Principle
High-level policy should not depend directly on low-level details. A checkout workflow should depend on a PaymentGateway interface, not a specific HTTP client. Laravel dependency injection makes this practical. Bind the interface to a concrete implementation in a service provider, and tests can replace it with a fake.
Using SOLID without overengineering
SOLID does not mean every class needs an interface. Start with clear responsibilities. Introduce interfaces where variation, testing, or external dependencies create pressure. Use patterns when the code asks for them. The best SOLID design feels boring because each class does exactly what its name promises.
Internal reading path
This article is part of a connected OOP and design patterns series. Continue with these related guides:
- Single Responsibility Principle in Laravel Services
- Open Closed Principle With PHP Strategies
- Dependency Injection in Laravel: Writing Flexible Services
Practical checklist
- Name the responsibility before choosing a pattern.
- Prefer small contracts where behavior varies or external services are involved.
- Keep controllers at the edge and move workflows into named application code.
- Add tests around the behavior that is most likely to change.
- Use patterns to reduce coupling, not to make simple code look advanced.