The application of SOLID principle in PHP includes five core points: 1. The single responsibility principle (SRP) requires each class to be responsible for only one task, and improve maintainability through separation functions such as UserService, UserRepository and EmailService; 2. The opening and closing principle (OCP) emphasizes extending openness, modifying closing, and using interfaces or abstract classes to implement new functions without changing old code. For example, the PaymentMethod interface supports multiple payment methods; 3. The Richter replacement principle (LSP) ensures that subclasses can replace parent classes without destroying logic and avoid behavior abnormalities in the inheritance tree, such as Square should not inherit Rectangle; 4. The interface isolation principle (ISP) advocates splitting large interfaces into small but specific interfaces, such as UserAuthInterface and UserProfileInterface to reduce redundant dependencies; 5. The Dependency Inversion Principle (DIP) advocates the dependency abstraction of high-level and underlying modules, and realizes loosely coupled design through dependency injection containers such as PHP-DI or Laravel IoC. Together, these principles promote clear, flexible and easy to scale.

SOLID principles are a set of five design principles intended to make software designs more understandable, flexible, and maintainable. They apply broadly across object-oriented programming languages, including PHP. These principles help developers avoid code smells, reflector effectively, and build systems that are easier to scale and test.

Single Responsibility Principle (SRP)

Each class should have only one reason to change — in other words, a class should do just one thing.

In PHP development, this means separating concerns. For example, instead of having a User class that handles user data, database operations, and email notifications, break it into separate classes like UserService , UserRepository , and EmailService .

Tips for applying SRP:

• Group methods around clear responsibility.
• Avoid putting unrelated logic inside the same class.
• Use services or utility classes to handle specific tasks.

This helps with testing, debugging, and future changes because you're not dealing with massive, tangled classes.

Open/Closed Principle (OCP)

Software entities (classes, modules, functions) should be open for extension but closed for modification.

In practice, this means writing code that can accept new features without changing existing code. In PHP, this often involves using interfaces or abstract classes to define contracts that multiple implementations can follow.

For instance, imagine you have a PaymentProcessor class. Instead of modifying it every time you add a new payment method (like PayPal or Stripe), create an interface called PaymentMethod and implement it separately for each provider.

How to approach this in PHP:

• Use interfaces or abstract classes as extension points.
• Design your system so new behavior comes from adding new code, not editing old code.
• This reduces the risk of breaking existing functionality when adding features.

Liskov Substitution Principle (LSP)

Objects of a superclass should be replaced with objects of a subclass without breaking the application.

This is about making sure that inheritance makes logical sense. If you're using a child class anywhere a parent class is expected, the behavior shouldn't cause errors or unexpected results.

A classic example where this breaks is if you have a Rectangle class and a Square class that extends it. If setting width and height independently on Square causes bugs, then Square shouldn't inherit from Rectangle .

In PHP:

• Make sure overridden methods preserve the original behavior expectations.
• Don't force subclasses to override methods they don't need.
• Keep your inheritance tree meaningful and predictable.

Interface Segregation Principle (ISP)

Clients should not be forced to depend on interfaces they do not use.

Instead of creating large, general-purpose interfaces, split them into smaller, more specific ones. This way, classes only need to implement what's relevant to them.

In PHP, this might mean avoiding something like a UserActionsInterface with 20 methods and instead splitting it into UserAuthInterface , UserProfileInterface , etc.

Why it matters:

• Reduces unnecessary coupling between components.
• Makes it easier to manage different implementations.
• Keeps code clean by avoiding dummy method implementations.

Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions. Also, abstractions should not depend on details; details should depend on abstractions.

This principle encourages coding to interfaces rather than concrete implementations. It's especially useful in PHP when building loosely coupled applications.

For example, instead of directly instantiating a MySQLDatabase class inside a service, type-hint an interface like DatabaseInterface and inject it via constructor or setter.

To apply DIP effectively in PHP:

• Use dependency injection containers like PHP-DI or Laravel's IoC container.
• Type-hint against interfaces in your constructors or method parameters.
• Let external configuration decide which implementation to use at runtime.

These principles may seem abstract at first, but they become second nature with practice. Applying them thoughtfully in PHP projects improves code quality, simplifies maintenance, and supports long-term scalability. It's not about following rules blindly — it's about writing code that's easy to understand and adapt over time.

Basically that's it.

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