使对象可以像数组一样进行foreach循环，要求属性必须是私有

<?php
class sample implements Iterator {
    private $_items = array (0, 1, 2, 3 );
 
    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/>';
}
/**
**输出结果为
0=>0
1=>1
2=>2
3=>3
**/

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