Introduction to OOP with PHP (Classes, Encapsulation, Inheritance)

The content here is under the Attribution 4.0 International (CC BY 4.0) license

The following content aims to introduce the concepts of OOP (Object Oriented Programming), complementing these concepts the PHP language is used to demonstrate practical examples, but can be written in any programming language.

This is also a companion for the talk I gave on this subject (available in PT-BR).

Introduction

Object-oriented programming is a paradigm used in programming to approximate reality. In comparison to structured programming where each line of code executes a statement, object-oriented programming is distinguished by representing code in classes.

• Classes: represents a prototype of an object. To create an object it is necessary to define a class beforehand.
• Object is the concrete instance of a class, which has behaviors.

The UML has a unique diagram for this type of approach, the class diagram. This diagram aims to represent how the interaction between classes is in a system

One of the advantages of using OOP instead of structured programming is code reuse, also known as inheritance and polymorphism. On the other hand, structured programming reuses code through functions and nothing else, which in some scenarios makes it difficult to use the same logic. Such as a function that needs to behave in a way based on a parameter. Typically this approach in structured programming is addressed with a switch / case.

Classes

A class can be represented by its behavior (methods) and attributes (variables) [1]

<?php

Class Person
{

   public $name;
   public $age;
   public $id;
}

In structured programming, this type of abstraction is done in a different way. For example in C is used struct to create a new data structure.

Classes in particular have two behaviors, the constructors and the destructors. Constructors are used to define an initial state of an object, and destructors are used to define the final behavior, and are executed before an object is destroyed from memory.

<?php

class Person {
  __construct() {}
  __destruct() {}
}

NOTE: methods that begin with __ in your name have a special meaning in PHP. These methods are known as magic methods, which have a certain behavior depending on the type of method invoked.

Encapsulation

Encapsulation provides a process of allowing / denying access to behavior and data that a given class has through the visibility modifiers.

NOTE: One of the first approaches of PHP (PHP 4) object orientation did not have implemented visibility. All attributes were public and accessible [2].

NOTE 2: With php 7.4, it is now possible to take properties of a class

PHP has three types of public, private, and protected visibility modifiers. Modifiers are used in the attributes and methods of a class and classes that inherit can modify the behavior of the parent class.

Andy Hunt and Dave Thomas touch on this point in a article published by the IEEE software in a nice and clean example, in which they elaborate on the enbugging term and as well as the code design [3].

Inheritance

Inheritance is known is the act of inheriting all behavior from one class to another, with the possibility of changing some particular desired behavior. The goal of inheritance is to reuse existing code.

Following with the example of the person class, in an actual application, could have the difference of type, such as for example physical or legal person.

<?php

class Person
{

   private $name;
   private $age;
   private $id;
}


class LegalPerson extends Person
{

   private $name;
   private $age;
   private $document;
}

Note: In this example we could go a little further and put a Person base class and create two other PersonPass and LegalPerson classes, so the cpf attribute would not be unusable in the legal class.

Polymorphism refers to many forms, in the context of object orientation, we apply polymorphism so that classes derived from other classes (inheritance) have the same methods but different behaviors.

This type of approach is used when programming for the parent class and not for a particular implementation.

<?php

Class Person
{
   private $name;
   private $age;
}


Class LegalPerson extends Person
{
   private $document;
}


Class NaturalPerson extends Person
{
   private $id;
}

In a scenario where we want only the document value independent of the type of person, polymorphism becomes important.

<?php

$naturalPerson = new NaturalPerson('john', 20, 3232323);
$legalPerson = new LegalPerson('maria' 1, 50, 999999);

$persons = [
  $naturalPerson,
  $legalPerson
];

This scenario illustrates the concept of abstraction and polymorphism, the final program, is concerned with the behaviors that exist in a person and not in specific behaviors of legal person or individual.

Even though we know what inheritance is, we should be careful when using it. The pragmatic programmer book [4] has a specific section in the chapter 5 (Bend or break) that elaborates on why we should avoid it.

Abstract classes

Abstract classes are used for this purpose, to create an abstraction based on some behavior. In one sense of class hierarchy, the abstract class is used as the basis for all others.

Note: At this point it is necessary to think exclusively about the hierarchy and how to use it, good programming practices, like testing, favor composition rather than inheritance.

<?php

abstract class Person {}

With abstract class it is not possible to instantiate an object, since the concrete class is allowed.

Final classes

On the other hand, sometimes it is possible that a scenario of blocking any type of inheritance is imposed. For this the final classes were there, declaring a class as final, it is not possible to extend it.

<?php

final class LegalPerson {}

Edit Oct 23, 2021

For more advanced usage of OOP, I would recommend to watch the talk Improving OO Design by Removing Old Habits as an introduction to Object Calisthenics [5].

References

1. [1]P. Dall’Oglio, “PHP Programando com Orientação a Objetos. Ed 1.,” 2007.
2. [2]H. Hayder, Object-Oriented Programming with PHP5. Packt, 2007 [Online]. Available at: https://subscription.packtpub.com/book/web_development/9781847192561/1/ch01lvl1sec09/difference-of-oop-in-php4-and-php5
3. [3]A. Hunt and D. Thomas, “The Art of Enbugging,” 2003 [Online]. Available at: https://www2.ccs.neu.edu/research/demeter/related-work/pragmatic-programmer/jan_03_enbug.pdf. [Accessed: 05-Jun-2021]
4. [4]D. Thomas and A. Hunt, The Pragmatic Programmer: your journey to mastery. Addison-Wesley Professional, 2019.
5. [5]R. Singham et al., The ThoughtWorks® Anthology: Essays on Software Technology and Innovation. Pragmatic bookshelf, 2008.