help with late static bindings

[azegurb]

hi i have read PHP pro patterns and desing book.
and i read this chapter but i didnt understand.

abstract class DomainObject { 
    private $group; 
    public function __construct() { 
        $this->group = static::getGroup(); 
    } 
 
    public static function create() { 
        return new static(); 
    } 
 
    static function getGroup() { 
        return "default"; 
    } 
} 
 
class User extends DomainObject { 
} 
 
class Document extends DomainObject { 
    static function getGroup() { 
        return "document"; 
    } 
} 
 
class SpreadSheet extends Document { 
}

this code outputs

print_r(User::create()); 
print_r(SpreadSheet::create());

i dont know for what reason

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

used
what is new static();
Thank you very much for attention