PracticeInheritance in object-oriented PHP

Practice: Inheritance in object-oriented PHP

The "Inheritance in object-oriented PHP" tutorial taught us how to use inheritance in order to reduce code duplication. For this aim, we extend the parent class by child classes that use the code from the parent, so we don't need to re-write the code repeatedly in each class. Instead, we need to write the code only once in the parent class and can use it in the child classes.

In the following section, we will be able to practice what we learned about inheritance in PHP by answering some questions and by writing our own code.

Tutorials list

Answer the question

We learned about three access control modifiers (public, private, and protected) that we use to allow/restrict access to the code.
In the following table, we will use "V" to mark that the code can be accessed from a certain level, and "X" if it cannot be accessed.
Fill in the table with the right values.
For example, public code can be accessed from within the class, from the code within the child classes, and from the global scope, while private code cannot be accessed from the global scope (*).

Modifier	Class	Child class	Global scope *
Public
Protected
Private

(*) The global scope is outside the classes.

Solution:

Modifier	Class	Child class	Global scope
Public
Protected
Private

Public methods and properties can be accessed from anywhere in the code.

Protected methods and properties can only be accessed from within the class and its child classes.

Private methods and properties can only be accessed from within the class itself.

Coding exercise

In the following example, we will create an Admin class, which is a child class of the User class.

Create a user class.

Solution:

class User { }

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

Add to the class a private property with the name of $username.

Solution:

class User {
  private $username;
}

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

Create a setter method that can set the value of the $username.

Solution:

class User {
  private $username;
 
  public function setUsername($name) 
  {
    $this -> username = $name;
  }
}

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

Create a class Admin that inherits the User class.

Solution:

class Admin extends User{}

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

Now, let's add to the Admin class some code. First, add a public method by the name of expressYourRole(), and make it return the string: "Admin".

Solution:

class Admin extends User {
  public function expressYourRole()
  {
    return strtolower(__CLASS__);
  }
}

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

Add to the Admin class another public method, sayHello(), that returns the string "Hello admin, XXX" with the username instead of XXX.

Solution:

class Admin extends User {
  public function expressYourRole()
  {
    return strtolower(__CLASS__);
  }
       
  public function sayHello()
  {
    return "Hello admin, " . $this -> username;
  }
}

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

Create an object $admin1 out of the class Admin
- set its name to "Balthazar" and say hello to the user.
Do you see any problem?

Expected result:
Notice: Undefined property: Admin::$username

Solution:

class User {
  private $username;
 
  public function setUsername($name) 
  {
    $this -> username = $name;
  }  
}
 
 
class Admin extends User {
  public function expressYourRole()
  {
    return strtolower(__CLASS__);
  }
       
  public function sayHello()
  {
    return "Hello admin, " . $this -> username;
  }
}
  
  
$admin1 = new Admin();
$admin1 -> setUsername("Balthazar");
$admin1 -> sayHello();

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

What do you think is the cause of the problem?

Solution:

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

Change the code to fix the problem.

Expected result:
Hello admin, Balthazar

Solution:

//We can declare the $usrname property as protected to allow the 
//   access for properties and methods that belongs to the child classes
class User {
  //Declare the $username as protected
  protected $username;
 
  public function setUsername($name) 
  {
    $this -> username = $name;
  }  
}
 
 
class Admin extends User {
  public function expressYourRole()
  {
    return strtolower(__CLASS__);
  }
      
  public function sayHello()
  {
    return "Hello admin, " . $this -> username;
  }
}
  
  
$admin1 = new Admin();
$admin1 -> setUsername("Balthazar");
echo $admin1 -> sayHello();

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

Try to write the solution with a getter method inside the parent that can be used from the child class.

Expected result:
Hello admin, Balthazar

Solution:

class User {
  private $username;
 
  public function setUsername($name) 
  {
    $this -> username = $name;
  }
      
  public function getUsername()
  {
    return $this -> username;
  }  
}
 
 
class Admin extends User{
  public function expressYourRole()
  {
    return strtolower(__CLASS__);
  }
      
  public function sayHello()
  {
    return "Hello admin, " . $this -> getUsername();
  }
}
  
  
$admin1 = new Admin();
$admin1 -> setUsername("Balthazar");
echo $admin1 -> sayHello();

Scratchpad to practice your coding *This will not be saved nor submitted to us.*

<?php
//Your practice code

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