Part 5 - Using Class Inheritance

Classes can inherit the methods and properties of another class using the extends keyword. For instance, to create a second class that extends MyClass and adds a method, you would add the following to your test file:

<?php

class MyClass

{

  public $prop1 = "I'm a class property!";

  public function __construct()

  {

      echo 'The class "', __CLASS__, '" was initiated!<br />';

  }

  public function __destruct()

  {

      echo 'The class "', __CLASS__, '" was destroyed.<br />';

  }

  public function __toString()

  {

      echo "Using the toString method: ";

      return $this->getProperty();

  }

  public function setProperty($newval)

  {

      $this->prop1 = $newval;

  }

  public function getProperty()

  {

      return $this->prop1 . "<br />";

  }

}

class MyOtherClass extends MyClass

{

  public function newMethod()

  {

      echo "From a new method in " . __CLASS__ . ".<br />";

  }

}

// Create a new object

$newobj = new MyOtherClass;

// Output the object as a string

echo $newobj->newMethod();

// Use a method from the parent class

echo $newobj->getProperty();

?>

Upon reloading the test file in your browser, the following is output:

The class "MyClass" was initiated!

From a new method in MyOtherClass.

I'm a class property!

The class "MyClass" was destroyed.

Overwriting Inherited Properties and Methods

To change the behavior of an existing property or method in the new class, you can simply overwrite it by declaring it again in the new class:

<?php

class MyClass

{

  public $prop1 = "I'm a class property!";

  public function __construct()

  {

      echo 'The class "', __CLASS__, '" was initiated!<br />';

  }

  public function __destruct()

  {

      echo 'The class "', __CLASS__, '" was destroyed.<br />';

  }

  public function __toString()

  {

      echo "Using the toString method: ";

      return $this->getProperty();

  }

  public function setProperty($newval)

  {

      $this->prop1 = $newval;

  }

  public function getProperty()

  {

      return $this->prop1 . "<br />";

  }

}

class MyOtherClass extends MyClass

{

  public function __construct()

  {

      echo "A new constructor in " . __CLASS__ . ".<br />";

  }

  public function newMethod()

  {

      echo "From a new method in " . __CLASS__ . ".<br />";

  }

}

// Create a new object

$newobj = new MyOtherClass;

// Output the object as a string

echo $newobj->newMethod();

// Use a method from the parent class

echo $newobj->getProperty();

?>

This changes the output in the browser to:

A new constructor in MyOtherClass.

From a new method in MyOtherClass.

I'm a class property!

The class "MyClass" was destroyed.

Preserving Original Method Functionality While Overwriting Methods

To add new functionality to an inherited method while keeping the original method intact, use the parent keyword with the scope resolution operator (::):

<?php

class MyClass

{

  public $prop1 = "I'm a class property!";

  public function __construct()

  {

      echo 'The class "', __CLASS__, '" was initiated!<br />';

  }

  public function __destruct()

  {

      echo 'The class "', __CLASS__, '" was destroyed.<br />';

  }

  public function __toString()

  {

      echo "Using the toString method: ";

      return $this->getProperty();

  }

  public function setProperty($newval)

  {

      $this->prop1 = $newval;

  }

  public function getProperty()

  {

      return $this->prop1 . "<br />";

  }

}

class MyOtherClass extends MyClass

{

  public function __construct()

  {

      parent::__construct(); // Call the parent class's constructor

      echo "A new constructor in " . __CLASS__ . ".<br />";

  }

  public function newMethod()

  {

      echo "From a new method in " . __CLASS__ . ".<br />";

  }

}

// Create a new object

$newobj = new MyOtherClass;

// Output the object as a string

echo $newobj->newMethod();

// Use a method from the parent class

echo $newobj->getProperty();

?>

This outputs the result of both the parent constructor and the new class's constructor:

The class "MyClass" was initiated!

A new constructor in MyOtherClass.

From a new method in MyOtherClass.

I'm a class property!

The class "MyClass" was destroyed.