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：
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 }
结果输出：
立即学习“PHP免费学习笔记（深入）”；
====================== 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。
