Inheritance

In the last lesson, we looked at visibility and the access modifiers. I showed you an example of public and private, but I couldn't fully explain protected because we hadn't learned about inheritance yet.

Inheritance is simply the ability to create a new class from an existing class. The new class is called a child class or a subclass. The existing class is called a parent class or a superclass. The child class inherits all of the properties and methods of the parent class. Let's create a new class called Admin that inherits from the User class:

class Admin extends User
{
    public $level;

    public function __construct($name, $email, $level)
    {
        $this->level = $level;
        parent::__construct($name, $email); // Calls the parent constructor
    }
}

We use the extends keyword to inherit a class. We added a constructor that takes in the same 2 properties that the User class takes in, plus a new property called level. We set the value of the level property to the value of the $level argument that is passed to the constructor. We also call the parent constructor using the parent keyword. This is necessary because the parent constructor sets the name and email properties.

Let's initialize a new Admin:

$admin1 = new Admin('Tom Smith','tom@gmail.com', 5);

We can access the properties and methods of the User class using the arrow operator:

echo $admin1->name; // Tom Smith
echo $admin1->email; // tom@gmail
$admin1->login(); // Tom Smith logged in.

Now watch this. This is where the protected access modifier comes in. Let's change the access modifier of the status property in the User class to protected:

protected $status;

Let's add a new method in the Admin class called showStatus:

public function showStatus() {
  echo $this->status;
}

Now you should see the status. However, if you try to access the status property directly, you will get an error:

echo $admin1->status; // Fatal error: Uncaught Error: Cannot access protected property Admin::$status

This is because you can only access protected properties and methods from within the class and any class that inherits from it.

If you change the modifier to private, you won't even be able to access it from within the child class. It is only accessible from within the parent class.

Overriding Methods

You can override methods in the child class. Let's override the login method in the Admin class:

public function login()
{
    echo 'Admin ' . $this->name . ' logged in.';
}

Now if you call the login method on the Admin instance, you will see the new message:

$admin1->login(); // Admin Tom Smith logged in.