Luhn算法​验证银行卡是否有效

function checkCard($card)
{
$num = 0;
$card = str_split(trim($card));
krsort($card);
$i = 1;
foreach($card as $val){
if ($i % 2) {//奇数
$num += $val;
} else {//偶数
$n = $val * 2;
if ($n > 9) $n -= 9;
$num += $n;
}
$i++;
}
return (($num % 10) == 0);
}