Explore key design principles and their impact on Test Driven Development (TDD) in this 40-minute PHP UK Conference talk. Dive into responsibility-driven design, "Tell don't ask" principle, the Law of Demeter, and Dependency Inversion. Gain insights into object-oriented programming, encapsulation, abstraction, and how to avoid inappropriate intimacy in code. Learn to make better architectural decisions, understand the importance of interfaces, and discover how to create a more flexible and maintainable codebase. Apply these concepts to improve your coding and testing practices, ultimately enhancing your ability to leverage TDD for better software design.