– 4WP plugins show SOLID in practice: separate products instead of one bloated monolith. – `do_action()` and `apply_filters()` implement OCP in procedural form. – The principles overlap: one architectural decision often satisfies several at once.

– Encapsulation, abstraction, inheritance and polymorphism are one foundation, not four separate rules. – OOP scales from a brochure site to an enterprise platform. – Once the four principles click, SOLID becomes the natural next step.

– No client should depend on methods it does not use. – Large interfaces are better split into narrow contracts. – Applies to WordPress widgets, post handlers and plugin APIs.

– A subtype must be able to replace its base type without breaking behaviour. – Violations are common in payment gateways, storage adapters and cache layers. – Following LSP makes abstractions trustworthy and modularity real.

– Plugins break from change, not from complexity. – Interfaces instead of `if` chains: each payment gateway lives in its own class. – The system grows through extension, not through new branches of code.

– A class should have only one reason to change. – Controller, Service, Repository and Integration layers instead of “utility” monoliths. – SRP is the base for enterprise work and technical interviews.