SOLID Principles in PHP – E-learning Project Example

This project demonstrates the SOLID principles of Object-Oriented Programming using a simple e-learning system built in raw PHP. These principles help create software that is modular, maintainable, and scalable.

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✅ S – Single Responsibility Principle (SRP)

A class should have one and only one reason to change.

Applied in:

• CourseUploader: Handles only uploading logic.
• CourseNotification: Responsible for sending notifications only.
• EmailNotifier, SMSNotifier: Each handles one task (sending email or SMS).

✅ This ensures that each class is focused and easy to test or change independently.

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✅ O – Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification.

Applied in:

• Add new types of NotifierInterface (e.g., SlackNotifier, PushNotifier) without changing existing code.
• Add new LessonInterface types like QuizLesson, AssignmentLesson.

✅ You can extend functionality without modifying tested code.

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✅ L – Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types.

Applied in:

• VideoLesson and TextLesson extend Lesson and override getContentLink() correctly.
• Wherever Lesson is expected, child classes work without breaking logic.

✅ Guarantees that inherited classes behave as expected.

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✅ I – Interface Segregation Principle (ISP)

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

Applied in:

• NotifierInterface has only one method: send($message).
• LessonInterface has only one method: getContentLink().

✅ Keeps interfaces clean and focused on specific contracts.

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✅ D – Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

Applied in:

• CourseNotification depends on NotifierInterface, not specific implementations.
• Easily switch between EmailNotifier, SMSNotifier, etc.

✅ Improves flexibility and testability.

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💡 Example Use Case (Summary)

// Upload a course
$uploader = new CourseUploader();
$uploader->uploadCourse("OOP in PHP");

// Notify users
$notifier = new CourseNotification(new EmailNotifier());
$notifier->notify("New course uploaded");

// Access content
$lesson = new VideoLesson();
echo $lesson->getContentLink();