写一段代码，找到所有子集合，如[a,b,c]的子集合有{},{a},{b},{c},{ab},{ac},{abc}

//递归法
function allSubSet(array $arr = [], $sub_set = "", $begin_point)
{
    $res = [];
    if($sub_set) {
        $res[] = "{" . trim($sub_set, ",") . "}";
    }
    for ($start_point = $begin_point; $start_point < count($arr); ++$start_point) {
        $res = array_merge(
            $res, 
            allSubSet($arr, $sub_set . "," . $arr[$start_point], $start_point + 1)
        );
    }

    return $res;
}

$data = allSubSet([1, 2, 3], '', 0);
var_dump($data);