PHP OOP - Inheritance

PHP - What is Inheritance?

Inheritance in OOP = When a class derives from another class.

The child class will inherit all the public and protected properties and methods from the parent class. In addition, it can have its own properties and methods.

An inherited class is defined by using the extends keyword.

Let's look at an example:

< ?php
class Fruit {
  public $name;
  public $color;
  public function __construct($name, $color) {
    $this->name = $name;
    $this->color = $color; 
  }
  public function intro() {
    echo "The fruit is {$this->name} and the color is {$this->color}."; 
  }
}

// Strawberry is inherited from Fruit
class Strawberry extends Fruit {
  public function message() {
    echo "Am I a fruit or a berry? "; 
  }
}
$strawberry = new Strawberry("Strawberry", "red");
$strawberry->message();
$strawberry->intro();
?>

The Strawberry class is inherited from the Fruit class.

This means that the Strawberry class can use the public $name and $color properties as well as the public __construct() and intro() methods from the Fruit class because of inheritance.

The Strawberry class also has its own method: message().

PHP - Inheritance and the Protected Access Modifier

In the previous chapter we learned that protected properties or methods can be accessed within the class and by classes derived from that class. What does that mean?

Let's look at an example:

< ?php
class Fruit {
  public $name;
  public $color;
  public function __construct($name, $color) {
    $this->name = $name;
    $this->color = $color; 
  }
  protected function intro() {
    echo "The fruit is {$this->name} and the color is {$this->color}."; 
  }
}

class Strawberry extends Fruit {
  public function message() {
    echo "Am I a fruit or a berry? "; 
  }
}

// Try to call all three methods from outside class
$strawberry = new Strawberry("Strawberry", "red");  // OK. __construct() is public
$strawberry->message(); // OK. message() is public
$strawberry->intro(); // ERROR. intro() is protected
?>

In the example above we see that if we try to call a protected method (intro()) from outside the class, we will receive an error. public methods will work fine!

Let's look at another example:

< ?php
class Fruit {
  public $name;
  public $color;
  public function __construct($name, $color) {
    $this->name = $name;
    $this->color = $color; 
  }
  protected function intro() {
    echo "The fruit is {$this->name} and the color is {$this->color}."; 
  }
}

class Strawberry extends Fruit {
  public function message() {
    echo "Am I a fruit or a berry? ";
    // Call protected method from within derived class - OK
    $this -> intro();
  }
}

$strawberry = new Strawberry("Strawberry", "red"); // OK. __construct() is public
$strawberry->message(); // OK. message() is public and it calls intro() (which is protected) from within the derived class
?>

In the example above we see that all works fine! It is because we call the protected method (intro()) from inside the derived class.

PHP - Overriding Inherited Methods

Inherited methods can be overridden by redefining the methods (use the same name) in the child class.

Look at the example below. The __construct() and intro() methods in the child class (Strawberry) will override the __construct() and intro() methods in the parent class (Fruit):

< ?php
class Fruit {
  public $name;
  public $color;
  public function __construct($name, $color) {
    $this->name = $name;
    $this->color = $color; 
  }
  public function intro() {
    echo "The fruit is {$this->name} and the color is {$this->color}."; 
  }
}

class Strawberry extends Fruit {
  public $weight;
  public function __construct($name, $color, $weight) {
    $this->name = $name;
    $this->color = $color;
    $this->weight = $weight; 
  }
  public function intro() {
    echo "The fruit is {$this->name}, the color is {$this->color}, and the weight is {$this->weight} gram."; 
  }
}

$strawberry = new Strawberry("Strawberry", "red", 50);
$strawberry->intro();
?>

PHP - The final Keyword

The final keyword can be used to prevent class inheritance or to prevent method overriding.

The following example shows how to prevent class inheritance:

< ?php
final class Fruit {
  // some code 
}

// will result in error
class Strawberry extends Fruit {
  // some code
}
?>

The following example shows how to prevent method overriding:

< ?php
class Fruit {
  final public function intro() {
    // some code
  }
}

class Strawberry extends Fruit {
  // will result in error
  public function intro() {
    // some code
  }
}
?>