What Is a Superclass and a Subclass

This section describes what is a Superclass and a Subclass - a pair of classes that the second class extends from the first class. An object of subclass inherits all properties and operations from the superclass.

What Is a Superclass and a Subclass? A superclass and a subclass is a pair of classes that the second class extends from the first class. An object of subclass inherits all properties and operations from the superclass.

In PHP, a subclass is declared with the keyword "extends" followed by the superclass name. For example, the following tutorial example provides a class MySquare which is extended from class MyRectangle. In this case, MyRectangle is the superclass and MySquare is subclass.

<?php 
/* MySquare.php
#- Copyright (c) 2015-2019, HerongYang.com, All Rights Reserved.
*/

class MyRectangle {
   public $width;
   public $height;

   public function __construct($w,$h) {
      $this->width = $w;
      $this->height = $h;
   }
   public function getArea() {
      return $this->width * $this->height;
   }
}

class MySquare extends MyRectangle {
   public $length;

   public function __construct($l) {
      parent::__construct($l,$l);
      $this->length = $l;
   }
   public function getArea() {
      return $this->length**2;
   }
}

  $object = new MySquare(2.0);
  
  echo "MySquare object: \n";
  echo "   Class name: ".  get_class($object) ."\n";
  echo "   Length: " . $object->length ."\n";
  echo "   Width: " . $object->width ."\n";
  echo "   Height: " . $object->height ."\n";
  echo "   Area: " . $object->getArea() ."\n";
  echo "   Is subclass of MyRectangle: " 
     . is_subclass_of($object, 'MyRectangle') ."\n";
  echo "   Is subclass of MySquare: " 
     . is_subclass_of($object, 'MySquare') ."\n";
?>

Notice that:

If you run the above tutorial example, you will get something like this:

C:\herong> \php\php MySquare.php

MySquare object:
   Class name: MySquare
   Length: 2
   Width: 2
   Height: 2
   Area: 4
   Is subclass of MyRectangle: 1
   Is subclass of MySquare:

Last update: 2019.

Table of Contents

 About This Book

 Introduction and Installation of PHP 7.3

 PHP Script File Syntax

 PHP Data Types and Data Literals

 Variables, References, and Constants

 Expressions, Operations and Type Conversions

 Conditional Statements - "if" and "switch"

 Loop Statements - "while", "for", and "do ... while"

 Function Declaration, Arguments, and Return Values

 Arrays - Ordered Maps

Introduction of Class and Object

 What Is a Class

 What Is an Object

 What Is a Constructor

 What Is a Static Method

 What Is a Static Variable

What Is a Superclass and a Subclass

 What Is an Abstract Class

 What Is an Abstract Method

 What Is an Interface

 What Is a Trait

 What Is an Overloaded Property

 What Is an Overloaded Method

 What Is Object Property Iteration

 What Is Object Cloning

 What Is Object Serialization

 What Is in an Object Variable

 Updating Variables Who Share an Object

 Passing Objects as Function Arguments

 Integrating PHP with Apache Web Server

 Retrieving Information from HTTP Requests

 Creating and Managing Sessions in PHP Scripts

 Sending and Receiving Cookies in PHP Scripts

 Controlling HTTP Response Header Lines in PHP Scripts

 MySQL Server Connection and Access Functions

 Functions to Manage Directories, Files and Images

 SOAP Extension Function and Calling Web Services

 SOAP Server Functions and Examples

 Localization Overview of Web Applications

 Using Non-ASCII Characters in HTML Documents

 Using Non-ASCII Characters as PHP Script String Literals

 Receiving Non-ASCII Characters from Input Forms

 "mbstring" Extension and Non-ASCII Encoding Management

 Managing Non-ASCII Character Strings with MySQL Servers

 Configuring and Sending out Emails

 Outdated Tutorials

 References

 Full Version in PDF/EPUB