I was just wondering if it's at all possible to create a service like Feed My Inbox on my own server using PHP?

Basically I have a site which has RSS feeds which are dynamic in nature and can search from thousands of posts based on many different criteria. I have the RSS feed working fine and bringing back data dynamically for whatever criteria I want so that bits fine.

I am using the ExpressionEngine CMS to handle the site and there will be thousands of users on the site (currently there are around 2,0000) but that number is exponentially growing every single day.

What I want to be able to do is allow the users to choose from certain criteria which will then build a dynamic RSS URL which will then be stored in a database table (one row for each user). This bit I will be able to do myself but then I want to be able to send out new RSS feed items via e-mail to each user.

This is the part I'm a little stuck on. I'm guessing I would somehow need to run a cron job to hit a page which would check each users RSS feed and then if there are new items to send them to the user via e-mail.

That's where I am totally stuck though and I'm just wondering what the best way to go about it would be? That or any software in PHP that already does this sort of thing would be great. I tried out phpList but it has severe problems working with RSS and I only ever got it to work once and now never again and I've read that lots of people have had this same problem so unfortunately it's not just me :-(

I know there are services such as Feed My Inbox which I could easily set up so that users click a link and their RSS feed URL is added to go and use that service but I want to keep users from seeing the dynamic nature of the feed or they will easily be able to modify it to get at other items in the feed.

I need this so that I can charge for access to the feeds but if people can see the URL of the feed then I will be totally unstuck as they will be able to get at whatever they want very easily.

Therefore I'd like to be able to send the items out to them.

Would really love to hear if anyone knows if this kind of thing is possible at all and what would be involved?