Some examples about SOLID Design Principles in PHP

This principle is about actors and high level architecture. A class should have one, and only one, reason to change.

This principle is about class design and feature extensions. A class should be open for extension, but closed for modification.

This principle is about subtyping and inheritance Derived classes must be substitutable for their base classes.

This principle is about business logic to clients communication. Many client-specific interfaces are better than one general-purpose interface.

This principle wires up all other four principles in a single circle. Depend upon abstractions. Do not depend upon concretions.