Gather together the things that change for the same reasons. Separate those things that change for different reasons.

A class or module (a set of related classes) should hide its implementation details (i.e. how exactly things are done) but have a well defined interface that both allows its usage by other classes (public methods) and extension via inheritance (

protected and public methods).

LSP, when defined classically, is the most complicated principle in SOLID. In fact, it's not that complicated.

It is about class hierarchies and inheritance. When you implement a new class extending from base one

The principle points that an interface should not define more functionality that is actually used at the same time. It is like

single responsibiltiy but for interfaces. In other words: if an interface does differnt things, break it into multiple more focused interfaces.

Dependency inversion basically states that a class should tell what it needs via interfaces but never get what it needs itself.

See [dependencies.md](https://github.com/samdark/yii2-cookbook/blob/master/book/dependencies.md) for more information.