RSS to Email Script in PHP

I have an existing script already that you can modify. The script works for an HTML page, I need it modified to work for an RSS feed. The script will check an RSS feed for updates and then send the new update info to an email address. The script should use a text file to keep a record of the URL and then only send the email if the URL hasn't already been added to the text file. The script will be run by CRON, I will set that up.