Object Oriented Programming in PHP by Jeffrey Way (Tutsplus)
S.O.L.I.D is an acronym for the first five object-oriented design(OOD) principles:
- S – Single-responsiblity principle
- O – Open-closed principle
- L – Liskov substitution principle
- I – Interface segregation principle
- D – Dependency Inversion Principle
Single-responsibility Principle
A class should have one and only one job.
Open-closed Principle
Objects or entities should be open for extension, but closed for modification. This simply means that a class should be easily extendable without modifying the class itself.
Liskov substitution principle
Let q(x) be a property provable about objects of x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T.
All this is stating is that every subclass/derived class should be substitutable for their base/parent class.
Interface segregation principle
A client should never be forced to implement an interface that it doesn’t use or clients shouldn’t be forced to depend on methods they do not use.
Dependency Inversion principle
Entities must depend on abstractions not on concretions. It states that the high level module must not depend on the low level module, but they should depend on abstractions.
Polymorphism
Polymorphism describes a pattern in object oriented programming in which classes have different functionality while sharing a common interface.
The beauty of polymorphism is that the code working with the different classes does not need to know which class it is using since they’re all used the same way. A real world analogy for polymorphism is a button. Everyone knows how to use a button: you simply apply pressure to it. What a button “does,” however, depends on what it is connected to and the context in which it is used — but the result does not affect how it is used. If your boss tells you to press a button, you already have all the information needed to perform the task.
In the programming world, polymorphism is used to make applications more modular and extensible. Instead of messy conditional statements describing different courses of action, you create interchangeable objects that you select based on your needs. That is the basic goal of polymorphism.
For example, integers and floats are implicitly polymorphic since you can add, subtract, multiply and so on, irrespective of the fact that the types are different.
Encapsulation
Encapsulation refers to the state of objects - objects encapsulate their state and hide it from the outside; outside users of the class interact with it through its methods, but cannot access the classes state directly. So the class abstracts away the implementation details related to its state.