php 银行卡号验证是否正确

public function check_bankCard($card_number){
    $arr_no = str_split($card_number);
    $last_n = $arr_no[count($arr_no)-1];
    krsort($arr_no);
    $i = 1;
    $total = 0;
    foreach ($arr_no as $n){
        if($i%2==0){
            $ix = $n*2;
            if($ix>=10){
                $nx = 1 + ($ix % 10);
                $total += $nx;
            }else{
                $total += $ix;
            }
        }else{
            $total += $n;
        }
        $i++;
    }
    $total -= $last_n;
    $x = 10 - ($total % 10);
    if($x == $last_n){
        return true;
    }else{
        return false;
    }
}