PHP从父构造函数中构造的子类中获取父类变量

Question

With the following code snippet, why does $this->foo return NULL in the B child class constructor? From what I've read I was under the impression that a child class inherits all its parent variables and methods

$a = new A();

class A {
    protected $foo;

    public function __construct() {
        $this->foo = "Hello World";

        $b = new B();
    }
}

class B extends A {
    public function __construct() {
        var_dump($this->foo);
    }
}

The selected answer of this question seems to suggest $this->foo should be accessible.

Is it because B is being constructed from within its parent class? If that is the case, how can I access the variable in the child class?

Answer 1

You're doing new A() and then separately you're doing new B() . You have two entirely independent object instances. It doesn't matter that one instantiation happens in the constructor of A , that's entirely irrelevant. It also doesn't matter that B extends A . You have two independent objects, and object instances do not share data implicitly, which is what you seem to be expecting.

Since B 's constructor overrides its parent's constructor, it's not executing any of A 's code either, so essentially nothing is happening when you instantiate a new B .