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SOLID Principles

This project demonstrates the SOLID principles of object-oriented design, which are essential for creating maintainable and scalable software. Each principle is implemented in its respective PHP file.

Principles Overview

  1. Single Responsibility Principle (SRP):

  • "A class should have only one reason to change or have only one job/responsibility."

  • Example - In Single-Responsibility.php, the User class has a single responsibility of representing a user and other classes, such as UserRepository and UserNotifier, focus on their specific tasks.

  1. Open/Closed Principle (OCP):

  • "A class should be open for extensions but closed for modifications."

  • Example - In Open-Closed.php, the Shape class is open for extensions (adding new shapes) but closed for modifications (changing the behavior(methods) of existing shapes).

  1. Liskov Substitution Principle (LSP):

  • "Child or derived classes should be able to substitute their parent or base class without breaking the program's behavior."

  • Example -

    • In Liskov-Substitution.php, the Human class is an interface that can be defined with its child class Father or Child to implement the speak() method. Means child class should have all the abilities of parent class, if it can't do it, then it is against the Liskov Substitution Principle.

    • We could've also implemented the Father interface for Child class but Father could have some features which Child may not have.

  1. Interface Segregation Principle (ISP):

  • "Classes should not be forced to implement methods they do not use/need."

  • Example -

    • In Interface-Segregation.php, If the PrinterInterface having the scanDocument() method, It is not required, so we need to implement a separate interface the ScannerInterface for it.

    • Accrodingly MultiFunctionPrinter, SimplePrinter, SimpleScanner classes can implement the desired interfaces.

  1. Dependency Inversion Principle (DIP):

  • "High-level classes should not depend on low-level classes but rather on abstractions."

  • Example -

    • In Dependency-Inversion.php, UserService class is a High level class that does not directly depends on MysqlDatabase which is a low level class.

    • UserService class has a dependency on DatabaseInterface which is an abstraction of MysqlDatabase and PostgresqlDatabase classes.

Usage

To explore the implementation of each principle, refer to the corresponding PHP files in this directory. Each file contains examples and explanations of how the respective principle is applied.