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 <<<STR35 a: $this->a36 b: $this->b37 c: $this->c38 d: $this->dn39 STR;40 foreach($this->_properties as $k => $v){41 echo <<<STR42 Property $k: $vn43 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：
====================== 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。