Chapter 3. SOLID Design Principles

Building modular software requires strong knowledge of the class design. There are numerous guidelines out there, addressing the way we name our classes, number of variables they should have, what the size of methods should be, and so on. The PHP ecosystem managed to pack these into official PSR standard, more precisely PSR-1: Basic Coding Standard and PSR-2: Coding Style Guide. These are all general programming guidelines that keep our code readable, understandable, and maintainable.

Aside from programming guidelines, there are more specific design principles that we can apply during the class design. Ones that address the notions of low coupling, high cohesion, and strong encapsulation. We call them SOLID design principles, a term coined by Robert Cecil Martin in the early 2000s.

SOLID is an acronym for the following five principles:

• S: Single responsibility principle (SRP)
• O: Open/closed principle (OCP)
• L: Liskov substitution principle (LSP)
• I: Interface Segregation Principle (ISP)
• D: Dependency inversion principle (DIP)

Over a decade old, the idea of SOLID principles is far from obsolete, as they are at the heart of good class design. Throughout this chapter, we will look into each of these principles, getting to understand them by observing some of the obvious violations that break them.

In this chapter, we will be covering the following topics:

• Single responsibility principle
• Open/closed principle
• Liskov substitution principle
• Interface Segregation Principle
• Dependency inversion principle