SOLID is a set of five design principles that make code more maintainable, scalable, and testable. Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. Following them prevents code from becoming a tangled mess as projects grow.
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What are SOLID principles and why do they matter?
What is the Single Responsibility Principle?
A class should have only one reason to change—meaning it should only do one thing. A class that handles both user authentication and sending emails violates SRP and becomes hard to maintain.