RSS functioning problem

I need to create an RSS feed for our information system, which is written in PHP. I had no problems with the RSS 2.0 specification, nor with the creation of RSS feed generator. Items for the feed are to be fetched from a large table containing lots of records, so it will take a lot of time to get all the necessary information from this table. Therefore, it is necessary to implement the following scheme:

1. To show 5 latest items to new subscribers.
2. For the existing subscribers – to show only those items which have been added since their last view of the feed.

I have no problems with the first condition: I can simply use the LIMIT clause to limit the number of fetched rows. Something like this:

$items = fun开发者_Python百科ction_select(“SELECT * FROM some_table ORDER BY date DESC LIMIT 5);

But this creates the following problem: Suppose there are real feed subscribers who have already read the items from 1 up to 10. After they've been away for some period of time new items have been created; say, 10 new items.

During their next check-in we want them to see all the new 10 items, but not all at once. They will see only the last 5 ones (from 16 up to 20), but not all 10 of them. The items from 11 up to 15 will be omitted.

I suppose that in order to succeed in solving this problem there should be a kind of a flag to be sent to feed. For example: pubDate of the lasted fetched item. Twitter's feed uses something similar. However, that link is hand-made. How could it be done another way? Please let me know if you have any ideas. If you have any example ready (no matter in what language) just share a link with me. I would appreciate it greatly.

Thank you in advance.

Standard RSS feeds don't render different content to different users. They simply always provide the most recent few items (often 10), and rely on the RSS reader to poll them often enough that they don't miss any updates. Unless you have a particularly compelling reason not to do this, this is the simplest and most effective mechanism.