4.5 延迟静态绑定：static 关键字 《深入PHP：面向对象、模式与实践》

self：指的不是调用上下文，它指的是 解析上下文。

<?php
abstract class DomainObject {
public static function create() {
return new self();//这样会报错：
}
}
class User extends DomainObject {

}
class Document extends DomainObject {

}
print_r(Document::create());

报错信息： Fatal error: Cannot instantiate abstract class DomainObject in

如果是static就没有问题：

<?php
abstract class DomainObject {
public static function create() {
return new static();
}
}
class User extends DomainObject {

}

class Document extends DomainObject {

}

print_r(Document::create());

输出结果为：

-----------------------------------

Document Object ( )

----------------------

<?php
abstract class DomainObject {
private $group;
    public function __construct(){
    $this->group = static::getGroup();//需要是个静态方法
    }
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());
print_r(SpreadSheet::create());

运行结果：

-----------------------------

User Object ( [group:DomainObject:private] => default ) SpreadSheet Object ( [group:DomainObject:private] => Document )

----------------------------

<?php
class A {
    public static function who() {
        echo __CLASS__;
    }
    public static function test() {
        self::who();
    }
}

class B extends A {
    public static function who() {
        echo __CLASS__;
    }
}

B::test();//输出A