php有一類很神奇的方法，這些方法是保留方法，通常不會在外部被顯式調用，他們使用雙下劃線(__)開頭，他們被稱為魔術方法(Magic Methods)。php官方也不建議定義其他雙下劃線開頭的方法。
這次介紹屬性重載方法：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 )
這些方法觸發的時機，都是在訪問不可訪問的屬性的時候。
如以下：
1 <?php
2
3 class Cls{
4
5 private $a = 0;
6 protected $b = 1;
7 public $c = 2;
8 var $d = 3;
9
10 private $_properties = array(0);
11
12 public function __set($name, $value){
13 echo __METHOD__ . ' is called! ' . json_encode(func_get_args()) . "\n";
14 return $this->_properties[$name] = $value;
15 }
16
17 public function __get($name){
18 echo __METHOD__ . ' is called! ' . json_encode(func_get_args()) . "\n";
19 return $this->_properties[$name];
20 }
21
22 public function __isset($name){
23 echo __METHOD__ . ' is called! ' . json_encode(func_get_args()) . "\n";
24 return isset($this->_properties[$name]);
25 }
26
27 public function __unset($name){
28 echo __METHOD__ . ' is called! ' . json_encode(func_get_args()) . "\n";
29 unset($this->_properties[$name]);
30 }
31
32 public function dump(){
33 echo "====================== DUMP START ====================\n";
34 echo <<<STR
35 a: $this->a
36 b: $this->b
37 c: $this->c
38 d: $this->d\n
39 STR;
40 foreach($this->_properties as $k => $v){
41 echo <<<STR
42 Property $k: $v\n
43 STR;
44 }
45
46 echo "====================== DUMP STOP =====================\n";
47
48 }
49 }
50 $string = 'abcdef';
51 $obj = new Cls();
52
53 $list = array('isset', 'get', 'set', 'isset', 'unset', 'isset');
54 $obj->dump();
55 foreach($list as $method){
56 for($i = 0 ; $i < strlen($string) ; $i ++){
57 $char = $string{$i};
58 echo "------ $method : $char ------\n";
59 switch($method){
60 case 'isset':
61 echo json_encode($tmp = isset($obj->$char)) . "\n";
62 break;
63 case 'get':
64 echo json_encode($tmp = $obj->$char) . "\n";
65 break;
66 case 'set':
67 echo json_encode($tmp = $obj->$char = $char . "-val") . "\n";
68 break;
69 case 'unset':
70 unset($obj->$char);
71 break;
72 default:
73 break;
74 }
75 }
76 $obj->dump();
77 }
結果輸出：
====================== 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 =====================
特別強調，當這個屬性不存在(基於當前訪問權限)的時候，才會觸發這些方法。因此嚴重建議使用確定的鍵值進行get/set處理，特別是對應的鍵值不要有訪問權限問題，如上例中的a和b，會導致在類內部、類外部、子類內處理的方式不同，開發時產生不可預知的返回值，極易產生bug。
empty方法不能調用屬性重載方法。
$tmp = $obj->$char = $char . "-val"
此例中，__set自身的返回值將被忽略，同樣__get也不會被調用。
此外：屬性重載方法必須聲明為public，同時參數不能使用引用傳遞，靜態屬性不能通過這些方法重載，這些方法也不能被聲明為static。
當然，使用魔術方法的時候，也會對反射系統造成影響。
