AchillesKal / solid-php

SOLID Principles PHP Examples

An introduction to S.O.L.I.D Principles using PHP

The term SOLID is a mnemonic acronym for five design principles intended to make software designs more understandable, flexible and maintainable.

S for Single Responsibility Principle(S.R.P)

A class should have one, and only one, reason to change.(A responsibility is a reason to change)

• Things that change for the same reasons should be together.
• Things that change for different reasons should not be together.

Symptoms of S.R.P violation:

• The class has many instance variables.
• The class has many public methods.
• Each method of the class uses other instance variables
• Specific tasks are delegated to private methods
• Rigidity: In order to change something, changes in multiple places should be applied.
• Tight coupling: Classes depend tightly on other classes, this leads to fragility.
• Fragility: One change breaks something completely different.

O for Open/closed principle

You should be able to extend a class’s behavior, without modifying it.

L for Liskov substitution principle

Derived classes must be substitutable for their base classes.

I for Interface segregation principle

Make fine-grained interfaces that are client specific

D for Dependency inversion principle

High-level modules should not depend on low-level modules. Both should depend on abstractions.