function DateDiff($date1, $date2, $unit = "") { //时间比较函数，返回两个日期相差几秒、几分钟、几小时或几天
    switch ($unit) {
        case 's':
            $dividend = 1;
            break;
        case 'i':
            $dividend = 60; //oSPHP.COM.CN
            break;
        case 'h':
            $dividend = 3600;
            break;
        case 'd':
            $dividend = 86400;
            break; //开源OSPhP.COM.CN
        default:
            $dividend = 86400;
    }
    $time1 = strtotime($date1);
    $time2 = strtotime($date2);
    if ($time1 && $time2)
        return (float)($time1 - $time2) / $dividend;
    return false;
}