class Father {
  public static function getSelf() {
    return new self();
  }

  public static function getStatic() {
    return new static();
  }
}

class Son extends Father {}

echo get_class(Son::getSelf()); // Father
echo get_class(Son::getStatic()); // Son
echo get_class(Father::getSelf()); // Father
echo get_class(Father::getStatic()); // Father

public static function getSelf() {
  return new self(); // new 關鍵字在 Father 這里
}