__sleep()方法通常用来在保存数据之前指定哪些属性需要被序列化。如果对象中包含一些完全不需要序列化的巨大对象，__sleep()就能派上用场了。
示例代码如下：
<?php
class Person
{
public $sex;
public $name;
public $age;
public function __construct($name="", $age=25, $sex='Male')
{
$this->name = $name;
$this->age = $age;
$this->sex = $sex;
}
/**
* @return array
*/
public function __sleep() {
echo "It is called when the serialize() method is called outside the class.<br>";
$this->name = base64_encode($this->name);
return array('name', 'age'); // 返回值中的元素必须是属性的名称
}
}
$person = new Person('John'); // Initially assigned.
echo serialize($person);
echo '<br/>';
输出结果如下：
It is called when the serialize() method is called outside the class.
O:6:"Person":2:{s:4:"name";s:8:"5bCP5piO";s:3:"age";i:25;}
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