php魔术方法——属性重载方法

php有一类很神奇的方法，这些方法是保留方法，通常不会在外部被显式调用，他们使用双下划线(__)开头，他们被称为魔术方法(Magic Methods)。php官方也不建议定义其他双下划线开头的方法。

这次介绍属性重载方法：get/set/isset/unset

public void __set ( string $name , mixed $value )

public mixed __get ( string $name )

public bool __isset ( string $name )

public void __unset ( string $name )

这些方法触发的时机，都是在访问不可访问的属性的时候。

如以下：

 <?php

 class Cls{

     private $a = 0;
     protected $b = 1;
     public $c = 2;
     var $d = 3;

     private $_properties = array(0);

     public function __set($name, $value){
         echo __METHOD__ . ' is called! ' . json_encode(func_get_args()) . "\n";
         return $this->_properties[$name] = $value;
     }

     public function __get($name){
         echo __METHOD__ . ' is called! ' . json_encode(func_get_args()) . "\n";
         return $this->_properties[$name];
     }

     public function __isset($name){
         echo __METHOD__ . ' is called! ' . json_encode(func_get_args()) . "\n";
         return isset($this->_properties[$name]);
     }

     public function __unset($name){
         echo __METHOD__ . ' is called! ' . json_encode(func_get_args()) . "\n";
         unset($this->_properties[$name]);
     }

     public function dump(){
         echo "====================== DUMP START ====================\n";
         echo <<<STR
     a: $this->a
     b: $this->b
     c: $this->c
     d: $this->d\n
 STR;
         foreach($this->_properties as $k => $v){
             echo <<<STR
     Property $k: $v\n
 STR;
         }

         echo "====================== DUMP STOP =====================\n";

     }
 }
 $string = 'abcdef';
 $obj = new Cls();

 $list = array('isset', 'get', 'set', 'isset', 'unset', 'isset');
 $obj->dump();
 foreach($list as $method){
     for($i = 0 ; $i < strlen($string) ; $i ++){
         $char = $string{$i};
         echo "------ $method : $char ------\n";
         switch($method){
             case 'isset':
                 echo json_encode($tmp = isset($obj->$char)) . "\n";
                 break;
             case 'get':
                 echo json_encode($tmp = $obj->$char) . "\n";
                 break;
             case 'set':
                 echo json_encode($tmp = $obj->$char = $char . "-val") . "\n";
                 break;
             case 'unset':
                 unset($obj->$char);
                 break;
             default:
                 break;
         }
     }
     $obj->dump();
 }

结果输出：

====================== DUMP START ====================
    a: 0
    b: 1
    c: 2
    d: 3
    Property 0: 0
====================== DUMP STOP =====================
------ isset : a ------
Cls::__isset is called! ["a"]
false
------ isset : b ------
Cls::__isset is called! ["b"]
false
------ isset : c ------
true
------ isset : d ------
true
------ isset : e ------
Cls::__isset is called! ["e"]
false
------ isset : f ------
Cls::__isset is called! ["f"]
false
====================== DUMP START ====================
    a: 0
    b: 1
    c: 2
    d: 3
    Property 0: 0
====================== DUMP STOP =====================
------ get : a ------
Cls::__get is called! ["a"]
null
------ get : b ------
Cls::__get is called! ["b"]
null
------ get : c ------
2
------ get : d ------
3
------ get : e ------
Cls::__get is called! ["e"]
null
------ get : f ------
Cls::__get is called! ["f"]
null
====================== DUMP START ====================
    a: 0
    b: 1
    c: 2
    d: 3
    Property 0: 0
====================== DUMP STOP =====================
------ set : a ------
Cls::__set is called! ["a","a-val"]
"a-val"
------ set : b ------
Cls::__set is called! ["b","b-val"]
"b-val"
------ set : c ------
"c-val"
------ set : d ------
"d-val"
------ set : e ------
Cls::__set is called! ["e","e-val"]
"e-val"
------ set : f ------
Cls::__set is called! ["f","f-val"]
"f-val"
====================== DUMP START ====================
    a: 0
    b: 1
    c: c-val
    d: d-val
    Property 0: 0
    Property a: a-val
    Property b: b-val
    Property e: e-val
    Property f: f-val
====================== DUMP STOP =====================
------ isset : a ------
Cls::__isset is called! ["a"]
true
------ isset : b ------
Cls::__isset is called! ["b"]
true
------ isset : c ------
true
------ isset : d ------
true
------ isset : e ------
Cls::__isset is called! ["e"]
true
------ isset : f ------
Cls::__isset is called! ["f"]
true
====================== DUMP START ====================
    a: 0
    b: 1
    c: c-val
    d: d-val
    Property 0: 0
    Property a: a-val
    Property b: b-val
    Property e: e-val
    Property f: f-val
====================== DUMP STOP =====================
------ unset : a ------
Cls::__unset is called! ["a"]
------ unset : b ------
Cls::__unset is called! ["b"]
------ unset : c ------
------ unset : d ------
------ unset : e ------
Cls::__unset is called! ["e"]
------ unset : f ------
Cls::__unset is called! ["f"]
====================== DUMP START ====================
Cls::__get is called! ["c"]
Cls::__get is called! ["d"]
    a: 0
    b: 1
    c:
    d:
    Property 0: 0
====================== DUMP STOP =====================
------ isset : a ------
Cls::__isset is called! ["a"]
false
------ isset : b ------
Cls::__isset is called! ["b"]
false
------ isset : c ------
Cls::__isset is called! ["c"]
false
------ isset : d ------
Cls::__isset is called! ["d"]
false
------ isset : e ------
Cls::__isset is called! ["e"]
false
------ isset : f ------
Cls::__isset is called! ["f"]
false
====================== DUMP START ====================
Cls::__get is called! ["c"]
Cls::__get is called! ["d"]
    a: 0
    b: 1
    c:
    d:
    Property 0: 0
====================== DUMP STOP =====================

特别强调，当这个属性不存在(基于当前访问权限)的时候，才会触发这些方法。因此严重建议使用确定的键值进行get/set处理，特别是对应的键值不要有访问权限问题，如上例中的a和b，会导致在类内部、类外部、子类内处理的方式不同，开发时产生不可预知的返回值，极易产生bug。

empty方法不能调用属性重载方法。

$tmp = $obj->$char = $char . "-val"

此例中，__set自身的返回值将被忽略，同样__get也不会被调用。

此外：属性重载方法必须声明为public，同时参数不能使用引用传递，静态属性不能通过这些方法重载，这些方法也不能被声明为static。

当然，使用魔术方法的时候，也会对反射系统造成影响。