RSS is one the hottest technologies at the moment, and even big web publishers (such as the New York Times) are getting into RSS as well. However, there are still a lot of websites that do not have RSS feeds.
If you still want to be able to check those websites in your favourite aggregator, you need to create your own RSS feed for those websites. This can be done automatically with PHP, using a method called screen scrapping. Screen scrapping is usually frowned upon, as it's mostly used to steal content from other websites.
I personally believe that in this case, to automatically generate a RSS feed, screen scrapping is not a bad thing. Now, on to the code!