Variables in Classes

Question

I stumbled across a very wired error in php:

class A {
     public $var = "test";
     public function __construct() {
        $this->var = "test2";
        $b = new B;
        $b->method();
    }
}

class B extends A {
    public function method() {
        $c = new C;
        $c->method();
    }
}

class C extends B {
    public function method() {
        echo $this->var;
    }
}

$a = new A;

I get the output "test", but I do not know why, cause the variable var should be overwritten in Class A.

If I output $var in Class A it says "test2", if I output it in Class B it says "test"…

Answer 1

The code on your question won't work because of the circular references (eg: $b = new B in A's constructor), which will cause PHP to run out of memory. You really shouldn't be instantiating children classes in a parent class.

That being said, by what you are describing, it sounds like you are defining a constructor in B, which overrides the parent constructor. In PHP children classes don't implicitly call the parent constructor (unlike in languages like Java).

So, it just inherits the original value for $var (ie: "test"), which is never changed. If you are overriding __construct() in B, you'll have to explicitly call the parent constructor, like:

class B extends A {
    public function __construct() {
        parent::__construct();
    }
}

And that should give you "test2" when you do something like:

$b = new B;
echo $b->var;

See this demo: http://ideone.com/Q9Bp8

---

What is the best way to have 3 classes, where the third and second can access variables of the first class?

The answer is, it depends on what you are doing. It sounds like you are not understanding how OOP works, which is a bigger problem. In general you only use inheritance when the children classes could reuse code from the parent class, and/or there is some sort of is-a or has-a relationship.

If your classes don't fit this model, just make the 3 classes independent, and hold a reference to the first class in your other classes. For example:

class A {
    public $n = 0;
    public function change($n) {
        $this->n = $n;
    }
}

class B {
    public function __construct($a) {
        $this->my_a = $a;
    }

    public function get() {
        return $this->my_a->n;
    }
}

$a = new A();
$b = new B($a):

echo $b->get();  // 0
$a->change(10);
echo $b->get();  // 10

See this demo: http://codepad.org/xL1Dzs0W