Difference between extend a class and create instance in another class in php

Question

First file:

class Class_one {

    public function __construct() {

    }

    public function show_name() {
        echo "My Name is Siam";
    }

    public function show_class() {
        echo "I read in class 7";
    }

}

Second file:

<?php

require_once './Class_one.php';

class Class_two {

    public $class_one;

    public function __construct() {
        $aa = new Class_one();
        $this->class_one = $aa;
    }

    public function history() {
        $this->class_one->show_name();
    }

}

$ab = new Class_two();
$ab->history();

What will happen if i don't instantiate class_one in class_two and extend class_one into class_two? Is there any difference?

score 2 · Answer 1 · answered Dec 25 '16 at 21:30

2

There's only one potential gotcha here: Class_two won't have direct access to Class_one private/protected methods and variables. That can be a good thing (encapsulation is where a class does some specific task and you don't want children to interfere with it), but it can also mean you can't get to the data you need.

I would suggest you do Class_two as a dependency injection (as shown in your example), however

class Class_two {

    public $class_one;

    public function __construct(Class_one $class) {
        $this->class_one = $class;
    }
}

answered Dec 25 '16 at 21:30

Machavity

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if i follow your suggestion then i need to pass class_one instance / object when i create instance of class_two. Am i right? – Md Zannatul Haque Siam Dec 25 '16 at 21:46
That's what I am suggesting. The only reason to instantiate it in the constructor as you showed, is that it's really an extension of the class (in which case it should be a parent class) – Machavity Dec 25 '16 at 21:47
the term " is that it's really an extension of the class (in which case it should be a parent class) " i don't understand :( – Md Zannatul Haque Siam Dec 25 '16 at 22:00
If two HAS to instantiate one in a specific way, then two should probably extend one – Machavity Dec 25 '16 at 23:06

score 1 · Answer 2 · edited May 23 '17 at 12:24

It should be a semantic difference:

You use inheritance when relationship between two classes can be expressed with to be verb. For example: My Ford is a Car
You use association/composition when relationship between two classes can be expressed with to have verb. For example: My car has an engine.

Above rules should be the first consideration when choosing inheritance or association in object-oriented programming. There've been always discussion about avoiding inheritance, and some will argue that you should choose composition over inheritance. See this other Q&A: Prefer composition over inheritance?

In terms of pure coding, as @Machavity on his answer has already said, there could be accessibility limitations when using composition instead of inheritance, but I wouldn't decide which approach to use based on member accessibility.

Difference between extend a class and create instance in another class in php

2 Answers2