PHP - Inheritance - php

Question

I am currently learning OOP concepts. I have used CodeIgniter, I know it has OOP concepts but I don't understand how it works. I just use the methods in the documentation.
I am now on the inheritance part.
Here is my code:
<?php
class Artist {
public $name;
public $genre;
public $test = 'This is a test string';
public function __construct(string $name, string $genre) {
$this->name = $name;
$this->genre = $genre;
}
}
class Song extends Artist {
public $title;
public $album;
public function __construct(string $title, string $album) {
$this->title = $title;
$this->album = $album;
}
public function getSongArtist() {
return $this->name;
}
}
$artist = new Artist('Joji Miller', 'Lo-Fi');
$song = new Song('Demons', 'In Tounges');
echo $song->getSongArtist(); // returns nothing
From what I understand inheritance will let me access properties and methods from the parent class.
In my example I have instantiate the Artist. So now I have Joji Miller as artist name.
Now, if I instantiate the Song class, I thought that I can access the artist name since I am extending the Artist class. But it is just empty.
Would you help me understand why it is not getting the artist name?
Hope I explained myself clearly. Thankyou..

Answer 1

Heh. Learning "oop principles" from CodeIgniter is like going to North Korea to study democracy. And you have already learned the wrong things.
The extends keyword should be read as "is special case of". As in class Admin extends User means that the admin instance is a more specialized case of generic user.
And that's where you are wrong. Song is not a subtype of an artist.
Instead the song has an artist, which performs it. As in:
$artist = new Artist('Freddie Mercury');
$song = new Song('Another One Bites the Dust', $artist);
echo $song->getArtist()->getName();
Another bad practice, that you seem to have picked up: stop defining class variables as public. That breaks the encapsulation. Instead those values should be assigned using methods, because then you will e able top track, format and validate those values.

Answer 2

First of all in your case you have not the best example of inheritance...
And it leads to confusion...
I'd rather advise you to have in base class base behavior related to all descendants, like here.
Base class:
<?php
class SomethingWithName
{
private $name;
public function __construct(string $name)
{
$this->name = $name;
}
public function getName(): string
{
return $this->name;
}
}
Your classes:
class Artist extends SomethingWithName
{
private $genre;
public function __construct(string $name, string $genre)
{
parent::__construct($name);
$this->genre = $genre;
}
public function getGenre(): string
{
return $this->genre;
}
}
class Song extends SomethingWithName
{
private $album;
private $artist;
public function __construct(string $name, string $album, Artist $artist)
{
parent::__construct($name);
$this->album = $album;
$this->artist = $artist;
}
public function getAlbum(): string
{
return $this->album;
}
public function getArtist(): Artist
{
return $this->artist;
}
}
Result:
$a = new Artist('Joji Miller', 'Lo-Fi');
$s = new Song('Demons', 'In Tounges', $a);
var_export([
$s->getName(), // Demons
$s->getAlbum(), // In Tounges
$s->getArtist()->getName(), // Joji Miller
$s->getArtist()->getGenre(), // Lo-Fi
]);

Answer 3

That's because when defining the Song class's __construct() function you are overriding the Artist's __contruct() function. Which is the function that gets called when you do $song = new Song(...)
Using the keyword parent:: you can access the parent's classes functions.
The following works.
<?php
class Artist {
public $name;
public $genre;
public $test = 'This is a test string';
public function __construct(string $name, string $genre) {
$this->name = $name;
$this->genre = $genre;
}
}
class Song extends Artist {
public $title;
public $album;
public function __construct(string $title, string $album, string $ArtistName, string $genre) {
$this->title = $title;
$this->album = $album;
parent::__construct($ArtistName, $genre);
}
public function getSongArtist() {
return $this->name;
}
}
$artist = new Artist('Joji Miller', 'Lo-Fi');
$song = new Song('Demons', 'In Tounges','Joji Miller', 'Lo-Fi');
echo $song->getSongArtist(); // returns nothing