php对不能使用foreach遍历的数据，通过实现Iterator接口来实现foreach遍历

必须实现5个方法，rewind, key, current, next, valid

class TestIterator implements Iterator {
    private $start, $end;
    private $cur;
    public $callback;

    public function __construct($start, $end, $callback) {
        $this->start = $start;
        $this->end = $end;
        $this->callback = $callback;
    }

    /**
     * 重新把迭代器指向列表的开始处
     *
     * @param
     *
     * @return
     */
    public function rewind() {
        $this->cur = $this->start;
    }
    
    /**
     * 返回当前位置的关键字
     *
     * @param
     *
     * @return
     */
    public function key() {
        return $this->cur;
    }

    /**
     * 返回当前位置的关键字
     *
     * @param
     *
     * @return
     */
    public function current() {
        return call_user_func_array($this->callback, array($this->cur));
    }

    /**
     * 把迭代器移动到下一个关键字
     *
     * @param
     *
     * @return
     */
    public function next() {
        $this->cur++;
        // $this->cur = $this->cur + 2;
    }

    /**
     * 返回true/false，判断是否还有更多的值（在调用current()和key()之前使用)
     *
     * @param
     *
     * @return
     */
    public function valid() {
        return $this->cur <= $this->end;
    }
}

$test = new TestIterator(3, 7, function($key) {
    return $key * 3 + 1;
});

foreach($test as $val) {
    echo $val . "</br>";
}