关于PHP的DateAdd自定义函数问题！

function DateAdd ($interval, $number, $date) {
    $date_time_array = getdate($date);
    $hours = $date_time_array["hours"];
    $minutes = $date_time_array["minutes"];
    $seconds = $date_time_array["seconds"];
    $month = $date_time_array["mon"];
    $day = $date_time_array["mday"];
    $year = $date_time_array["year"];
    switch ($interval)
    {
    case "yyyy": $year +=$number; break;
    case "q": $month +=($number*3); break;
    case "m": $month +=$number; break;
    case "y":
    case "d":
    case "w": $day+=$number; break;
    case "ww": $day+=($number*7); break;
    case "h": $hours+=$number; break;
    case "n": $minutes+=$number; break; //分钟
    case "s": $seconds+=$number; break;
    }
    $timestamp = mktime($hours ,$minutes, $seconds,$month ,$day, $year);
    return $timestamp;
}
$date_QT="8:21:12"
$date_Ten=DateAdd ("n", 10, $date_QT);//获得签到时间往后加10分钟的时间

我想返回一个为“8:31:12”这个时间行不！

但是显示“608”不知道为什么。望大家指教！！