$this->foo1();

public function foo1()
    {
        $arr9 = [0, 1, 2, 3, 4];
        \a_y::debug($this->foo2($arr9));
    }

public function foo2($arr)
    {
        static $result = [];
        $len = count($arr);
        if ($len < 1) {
            \a_y::debug($result);
            return $result;
        }
        $len --;
        $_a = rand(0, $len);
        array_push($result, $arr[$_a]);
        $new = [];
        foreach ($arr as $key=>$val){
            if ($val !== $arr[$_a]) {
                array_push($new, $val);
            }
        }
        $this->foo2($new);
    }

为什么我在foo1 里面debug出来是个空数组，而在foo2里面debug出来是有数据的？