php面接問題--羊の総数を求めます

function calSum()
{
        static $year = 1;
        static $arr = array(1, 0, 0, 0, 0); 
        if ($year <= 20)
        {   
                $year++;
                for ($j=4; $j>=0; $j--)
                {   
                        if ($j > 0)
                        {   
                                $arr[$j] = $arr[$j-1];
                        }   
                }   
                $arr[0] = $arr[2]+$arr[4];
                calSum();
        }   
        return array_sum($arr);
}