PHP OOP - Inheritance
PHP - Inheritance
Inheritance in PHP OOP allows a child class to inherit all the public and protected properties and methods from a parent class. In addition, the child class can have its own properties and methods.
Note: Private methods of a parent class are not accessible to a child class.
An inherited class is defined with the
extends
keyword.
In the following example, 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():
Example
<?php
// Parent class
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.<br>";
}
}
// 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->intro();
$strawberry->message();
?>
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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
<?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
?>
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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.
The public
method will work fine!
Let's look at another example:
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 calls intro() (which is protected) from within the
derived class
?>
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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
<?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 "A $this->name is $this->color,
and the weight is $this->weight gram.";
}
}
$strawberry = new Strawberry("Strawberry", "red",
50);
$strawberry->intro();
?>
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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
<?php
final class Fruit {
// some code
}
//
will result in error
class
Strawberry extends Fruit {
// some code
}
?>
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The following example shows how to prevent method overriding:
Example
<?php
class Fruit {
final public function intro() {
// some code
}
}
class
Strawberry extends Fruit {
//
will result in error
public function intro() {
// some code
}
}
?>
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