php class extends, variable from other extended class

Question

I have a main class and two extended classes:

class Main {
    public $foo;
}

class A extends Main {
  public function setVar()
    {
      $foo = "test"; 
    }
}

class B extends Main {
  public function getVar()
    {
      return $this->foo;
    }
}

$A = new A;
$B = new B;

$A->setVar();

echo "result: ".$B->getVar();

But result ($B->getVar()) stays empty. I am clearly missing something simple here... Besides that, is this the way to go splitting long classes up in relevant subclasses?

$A and $B are different __instances__ of your classes. Each is completely independent of the other... any change to $A only affects $A and not $B. And $foo = "test"; only sets a local varaible called $foo in the setVar() method of A, not the class varaible $foo.... to do that you'd need $this->foo = "test"; — Mark Baker, Jun 16 '12 at 09:50
*(related)* [Learning PHP classes](http://stackoverflow.com/questions/2206387/learning-php-class/2206835#2206835) — Gordon, Jun 16 '12 at 09:58
By the way, given the flaws in understanding some OOP principles, it may help to point out that *class* A and *instance* $A are two different kind of entities, also regarding their name (they need not to be the same at all). You could use: `$instance1 = new A; $instance2 = new B;` — Cranio, Jun 16 '12 at 10:22

score 4 · Answer 1 · answered Jun 16 '12 at 10:31

4

change:

class A extends Main {
    public function setVar()
    {
     $foo = "test"; 
    }
}

to:

class A extends Main {
    public function setVar()
    {
     $this->foo = "test"; 
    }
}

Notice the $this keyword

answered Jun 16 '12 at 10:31

Vlad Balmos

3,372
19
34

score 2 · Accepted Answer · answered Jun 16 '12 at 10:26

Example of using Dependency Injection (DI) to share an instance of Main between to different class instances rather than trying to use inheritence

class Main {
    public $foo;
}

class A {

    protected $main;

  public function setVar($data)
    {
      $this->main->foo = $data;
    }

  public function __construct(Main $main)
    {
      $this->main = $main;
    }
}

class B extends Main {
    protected $main;

  public function getVar()
    {
      return $this->main->foo;
    }

  public function __construct(Main $main)
    {
      $this->main = $main;
    }
}

$M = new Main;
$A = new A($M);
$B = new B($M);

$A->setVar("test");

echo "result: ".$B->getVar();

score 2 · Answer 3 · answered Jun 16 '12 at 10:28

To further clarify things, it may help to think it in this way. Let's say your Main is a class that stands for a vehicle (and hence we rename it Vehicle), so class means that Vehicle defines the characteristics of a veicle but not a particular one to do operations with (we need an instance for that). Let's translate also your class A in Car and B in Truck: therefore they are specializations of your main class that, along with possessing every characteristic of a generic Vehicle, extend the concept of "vehicle" and point out specific behaviours and properties of Cars in general, and Trucks in general, but - and that's the point - don't reference a particular car or truck. We need instances for that.

class Vehicle
{
   public $numberOfTires;
}

class Car extends Vehicle
{
    function smashMe()
    {
        echo "Oops!";
    }
}

class Truck extends Vehicle
{
    function smashMe()
    {
        echo "More Oops!";
    }
}

From this point we can define particular instances (your Porsche, my Camaro... different cars) on which we may perform operations (calling methods) and set properties.

$genericVehicle = new Vehicle;
$myPorsche = new Car;
$yourCamaro = new Car;
$hisTruck = new Truck;
$herTruck = new Truck;

But every instance remains independent from the other.

score 1 · Answer 4 · answered Jun 16 '12 at 09:57

Both are different instances of the the same class.

Operations done on one instance, does not apply on any of the others (That's kinda the point of OOP).

To do what you want, You can have B extend A, and then perform both operations on a single instance of the B class.

php class extends, variable from other extended class

4 Answers4

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