在静态方法中创建当前类的对象 - PHP

Question

I have some problems with my class Model and other classes so I made this simple example to explain my problem :

class person{

  public static $a = "welcome";

  public function __construct(){
                               }

  public static function getobject()
  {
      $v = new person();
      return $v;
  }

}

class student extends person{

  public static $b = "World";

}

$st = student:getobject();//this will return person object but I want student object

echo $st->$b; // There is an error here because the object is not student

So I want to know what to write instead $v = new person(); to get the object of the last inherited class.