PHP中魔术方法和魔术常量使用总结
魔术方法(Magic methods)
PHP中把以两个下划线__开头的方法称为魔术方法，这些方法在PHP中充当了举足轻重的作用。 魔术方法包括：
-
__construct()，类的构造函数
-
__destruct()，类的析构函数
-
__call()，在对象中调用一个不可访问方法时调用
-
__callStatic()，用静态方式中调用一个不可访问方法时调用
-
__get()，获得一个类的成员变量时调用
-
__set()，设置一个类的成员变量时调用
-
__isset()，当对不可访问属性调用isset()或empty()时调用
-
__unset()，当对不可访问属性调用unset()时被调用。
-
__sleep()，执行serialize()时，先会调用这个函数
-
__wakeup()，执行unserialize()时，先会调用这个函数
-
__toString()，类被当成字符串时的回应方法
-
__invoke()，调用函数的方式调用一个对象时的回应方法
-
__set_state()，调用var_export()导出类时，此静态方法会被调用。
-
__clone()，当对象复制完成时调用
__construct()和__destruct()
构造函数和析构函数应该不陌生，他们在对象创建和消亡时被调用。例如我们需要打开一个文件，在对象创建时打开，对象消亡时关闭
class FileRead
{
protected $handle = NULL;
function __construct(){
$this->handle = fopen(...);
}
function __destruct(){
fclose($this->handle);
}
}
?>这两个方法在继承时可以扩展，例如：
class TmpFileRead extends FileRead
{
function __construct(){
parent::__construct();
}
function __destruct(){
parent::__destruct();
}
}
?>__call()和__callStatic()
在对象中调用一个不可访问方法时会调用这两个方法，后者为静态方法。这两个方法我们在可变方法（Variable functions）调用中可能会用到。
class MethodTest
{
public function __call ($name, $arguments) {
echo "Calling object method '$name' ". implode(', ', $arguments). "\n";
}
public static function __callStatic ($name, $arguments) {
echo "Calling static method '$name' ". implode(', ', $arguments). "\n";
}
}
$obj = new MethodTest;
$obj->runTest('in object context');
MethodTest::runTest('in static context');
?>__get()，__set()，__isset()和__unset()
class MethodTest
{
private $data = array();
public function __set($name, $value){
$this->data[$name] = $value;
}
public function __get($name){
if(array_key_exists($name, $this->data))
return $this->data[$name];
return NULL;
}
public function __isset($name){
return isset($this->data[$name])
}
public function unset($name){
unset($this->data[$name]);
}
}
?>__sleep()和__wakeup()
当我们在执行serialize()和unserialize()时，会先调用这两个函数。例如我们在序列化一个对象时，这个对象有一个数据库链接，想要在反序列化中恢复链接状态，则可以通过重构这两个函数来实现链接的恢复。例子如下：
class Connection
{
protected $link;
private $server, $username, $password, $db;
public function __construct($server, $username, $password, $db)
{
$this->server = $server;
$this->username = $username;
$this->password = $password;
$this->db = $db;
$this->connect();
}
private function connect()
{
$this->link = mysql_connect($this->server, $this->username, $this->password);
mysql_select_db($this->db, $this->link);
}
public function __sleep()
{
return array('server', 'username', 'password', 'db');
}
public function __wakeup()
{
$this->connect();
}
}
?>__toString()
// Declare a simple class
class TestClass
{
public function __toString() {
return 'this is a object';
}
}
$class = new TestClass();
echo $class;
?>这个方法只能返回字符串，而且不可以在这个方法中抛出异常，否则会出现致命错误。
__invoke()
调用函数的方式调用一个对象时的回应方法。如下
class CallableClass
{
function __invoke() {
echo 'this is a object';
}
}
$obj = new CallableClass;
var_dump(is_callable($obj));
?>__set_state()
调用var_export()导出类时，此静态方法会被调用。
class A
{
public $var1;
public $var2;
public static function __set_state ($an_array) {
$obj = new A;
$obj->var1 = $an_array['var1'];
$obj->var2 = $an_array['var2'];
return $obj;
}
}
$a = new A;
$a->var1 = 5;
$a->var2 = 'foo';
var_dump(var_export($a));
?>__clone()
当对象复制完成时调用。例如在设计模式详解及PHP实现：单例模式一文中提到的单例模式实现方式，利用这个函数来防止对象被克隆。
public class Singleton {
private static $_instance = NULL;
// 私有构造方法
private function __construct() {}
public static function getInstance() {
if (is_null(self::$_instance)) {
self::$_instance = new Singleton();
}
return self::$_instance;
}
// 防止克隆实例
public function __clone(){
die('Clone is not allowed.' . E_USER_ERROR);
}
}
?>魔术常量(Magic constants)
PHP中的常量大部分都是不变的，但是有8个常量会随着他们所在代码位置的变化而变化，这8个常量被称为魔术常量。
-
__LINE__，文件中的当前行号
-
__FILE__，文件的完整路径和文件名
-
__DIR__，文件所在的目录
-
__FUNCTION__，函数名称
-
__CLASS__，类的名称
-
__TRAIT__，Trait的名字
-
__METHOD__，类的方法名
-
__NAMESPACE__，当前命名空间的名称
这些魔术常量常常被用于获得当前环境信息或者记录日志。
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