class GrandClass {
    public $data;
    public function __construct() {
        $this->someMethodInTheParentClass();
    }
    public function someMethodInTheParentClass() {
        $this->$data = 123456;
    }
}

class MyParent extends GrandClass{
    public function __construct() {
        parent::__construct();
    }
}

class Child extends MyParent {
    // public $data;
    public function __construct() {
        parent::__construct();
    }
    public function getData() {
        return $this->data;
    }
}

$a = new Child();
var_dump($a->getData());