<?php

 /*

  * __isset：当外部调用isset（）函数检测不可访问的属性或者不存在的属性时自动调用

    __unset：当在类外部调用unset销毁不可访问属性时自动调用

  * */

 class Person{

     private $userName;

     private $age;

     public $abc;

     public function __construct($userName,$age){

         $this->userName = $userName;

         $this->age = $age;

     }

     public function __isset($name){

         echo '当外部调用isset（）函数检测不可访问的属性或者不存在的属性时自动调用';

         return isset($name);

     }

     public function __unset($name){

         echo '当在类外部调用unset销毁不可访问属性时自动调用';

         unset($name);

     }

 }

 $p = new Person('Clive',15);

 var_dump(isset($p->abc));

 echo '<hr/>';

 unset($p->abc);