7.6 PHP 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:

Example 1: PHP Inheritance

<?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();
?>

Example Explained

• 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:

Example 2: PHP Inheritance - Inheritance and the Protected Access Modifier part 1

<?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:

Example 3: PHP Inheritance - Inheritance and the Protected Access Modifier part 2

<?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):

Example 4: PHP Inheritance - Overriding Inherited Methods

<?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:

Example 5: PHP Inheritance - The final Keyword

<?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:

Example 6: PHP Inheritance - The final Keyword part 2

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

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