zhangjingyu 
online.php是一个缓存文件，那么必然有一个文件会生成该文件。
经过搜索，应该是require/userglobal.php文件：
function addonlinefile($offset,$uid) {
    global $windid,$groupid,$lastvisit,$timestamp,$onlineip,$db_onlinetime,$fid,$tid,$wind_in,$tdtime, $db_olsize,$db_today;

    if (strlen($fid)>4) $fidwt = ''; else $fidwt = $fid;
    if (strlen($tid)>7) $tidwt = ''; else $tidwt = $tid;

    $wherebbsyou = getuseraction($fid,$wind_in);
    $acttime = get_date($timestamp,'m-d H:i');
    $D_name = "data/bbscache/online.php";
    if (!file_exists(D_P.$D_name)) {
        pwCache::writeover(D_P.$D_name,str_pad("",96)."\n");
    }
    if (GetCookie('hideid') != 1) {
        $newonline = "$windid\t$timestamp\t$onlineip\t$fidwt\t$tidwt\t$groupid\t$wherebbsyou\t$acttime\t$uid\t";
        $newonline = str_pad($newonline,$db_olsize)."\n";
        if (checkinline(D_P.$D_name,$offset,$windid)) {
            $isModify = 0;
            writeinline(D_P.$D_name,$newonline,$offset);
        } else {
            list($offset,$isModify)=GetInsertOffset(D_P.$D_name);
            writeinline(D_P.$D_name,$newonline,$offset);
        }
        if ($db_today && $timestamp-$lastvisit>$db_onlinetime) {
            require_once(R_P.'require/today.php');
        }
    } elseif (GetCookie('hideid') == 1) {
        require_once(R_P.'require/hidden.php');
    }
    if ($isModify === 1) {
        //频度可控制性
        ModifySelectFile(D_P."data/bbscache/guest.php");
    }
    return array($offset,$isModify);
}