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