function cartesian(array $arr, $str = [])
{
    $first = array_shift($arr);
    if (count($str) > 1) {
        foreach ($str as $k => $val) {
            foreach ($first as $key => $value) {
                $str2[] = $val . ',' . $value;
            }
        }
    } else {
        foreach ($first as $key => $value) {
            $str2[] = $value;
        }
    }
    if (count($arr) > 0) {
        $str2 = cartesian($arr, $str2);
    }
    return $str2;
}  

做生成商品sku的你时候，找了半天才找到的，特此记录一下