How do you override self? - php

Question

I want to build class that implement a lot of methods based on it's id,
so I want to have one class parent that implement the methods!
and when i want to use this class I will extend it and just override the id variable :
class parent
{
$id = "parent";
private __construct()
{
}
public static function create_instance()
{
$instance = new self();
return $instance;
}
public static function print_id()
{
echo $this->id;
}
}
class child extend parent
{
$id = "child";
}
$instance = child::create_instance();
$instance->print_id();
the result will be "parent", but I want the result to be child ?
How to do that ?
EDIT : I also tried this and got parent instead of child:
class parent1 {
private $id = "parent";
public function __construct() {
}
public static function create_instance() {
$instance = new static ();
return $instance;
}
public function print_id() {
echo $this->id;
}
}
class child extends parent1 {
private $id = "child";
}
$instance = child::create_instance ();
$instance->print_id ();

Answer 1

The problem is that the visibility of $id is private whereas it should be protected because print_id() is only defined on the parent; as such it can only reach its own $id.
class parent1 {
protected $id = "parent";
// ...
}
class child extends parent1 {
protected $id = "child";
}
The alternative is, of course, to override print_id() in the child class.

Answer 2

Currently when you call create_instance method on child class as a result instance of parent class is created not child class as you expect.
Use late static binding in parent class "create_instance" method:
public static function create_instance()
{
$instance = new static();
return $instance;
}
More details http://php.net/manual/en/language.oop5.late-static-bindings.php