php5.3 延迟静态绑定 static关键字

//传统模式 --这段代码能很好工作，但大量的重复代码很烦人,不想为每个DomainObject子类都创建这段相同代码吧？
/*

abstract class DomainObject{}

class User extends DomainObject
{
    public function __construct()
    {
        echo __METHOD__;
    }
    
    public static function create()
    {
        return new self(); 
    }

}
class Document extends DomainObject
{
    public function __construct()
    {
        echo __METHOD__;
    }
    
    public static function create()
    {
        return new self(); 
    }

}
User::create();
Document::create();
*/

//能否这么干？ 实例化最初调用者,谁调用实例化谁。但是这里self是指向的抽象类DomainObject
//fatal error: Cannot instantiate abstract class DomainObject 无法实例化抽象方法
/*
abstract class DomainObject
{
    public static function create()
    {
        return new self();
    }    
}

class User extends DomainObject{}
class Document extends DomainObject{}
User::create(); 
*/

//5.3后 可以使用static关键字，延迟静态绑定，通俗的就是，static 指向的是最初调用者
abstract class DomainObject
{
    public static function create()
    {
        return new static();
    }    
}
class User extends DomainObject
{
    public function __construct()
    {
        echo __METHOD__;
    }
    
}
class Document extends DomainObject
{
public function __construct()
    {
        echo __METHOD__;
    }
}
User::create(); //User::__construct
Document::create();  //Document::__construct

 /***********static 关键字不仅仅可以用于实例化，和self和parent一样，static还可以作为静态方法调用标识符**************/

abstract class DomainObject
{
    private $group;
    public function __construct()
    {
        //$this->group = self::getGroup();
        $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{}

print_r(User::create());         //     [group:DomainObject:private] => default
print_r(Document::create());     //     [group:DomainObject:private] => document
print_r(SpreadSheet::create());  //     [group:DomainObject:private] => document