关于PHP迭代器的一道笔试题

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我做事三分钟热度，却也爱你那么久。
### 问题 
1
使对象可以像数组一样进行foreach循环，要求属性必须是私有。

鸟哥给出的答案

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class sample implements Iterator
{
    private $_items = array(1,2,3,4,5,6,7);

    public function __construct() {
        ;//void
    }
    public function rewind()
    {
        reset($this->_items);
    }
    public function current()
    {
        return current($this->_items);
    }
    public function key()
    {
        return key($this->_items);
    }
    public function next()
    {
        return next($this->_items);
    }
    public function valid()
    {
        return ( $this->current() !== false );
    }
}

$sa = new sample();
foreach($sa as $key => $val){
    print $key . "=>" .$val.'<br>';
}

拓展说明

使用foreach只可以循环出数组的public属性，protected和private属性无法得到

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class User
{
    public $a = 'tom';
    protected  $b = 'lilei';
    private  $c = 'hanmeimei';
    public function __construct()
    {
    }
}

foreach (new User() as $user){
    echo $user.'<br>';
}

但是在类的内部可以

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class User
{
    public $a = 'tom';
    protected  $b = 'lilei';
    private  $c = 'hanmeimei';
    public function do_something() {
        foreach($this as $key => $value) {
            echo "$key => $value\n";
        }
    }
}

$user = new User();
$user->do_something();

http://www.laruence.com/2008/10/31/574.html