多进制加法

function hex_add($a, $b)
{
    $result = '';
    $chars = '0123456789abcdef';
    $len_a = strlen($a);
    $len_b = strlen($b);

    if ($len_a > $len_b) {
        $max = $len_a;
        $b = str_pad($b, $len_a, '0', STR_PAD_LEFT);
    } else {
        $max = $len_b;
        $a = str_pad($a, $len_b, '0', STR_PAD_LEFT);
    }

    $inc = $pos = 0;
    for ($i = $max - 1; $i >= 0; $i--) {
        $pos = hexdec($a[$i]) + hexdec($b[$i]) + $inc;
        $inc = floor($pos / 16);
        $result =  $chars[$pos % 16] . $result;
    }
    if ($inc) {
        return $inc . $result;
    }
    return $result;
}