You can create and customize your website's RSS or Atom feeds by generating digests with these online controls. Enter your URL(s) into Feed Scraper, work the controls, and...voila! Free and fee versions.
MagpieRSS provides an XML-based (expat) RSS parser in PHP.
MagpieRSS is compatible with RSS 0.9 through RSS 1.0. Also parses RSS 1.0's modules, RSS 2.0, and Atom. (with a few exceptions).
openpub - Project Hosting on Google Code. an application of the Atom Syndication Format intended to enable content creators and distributors to distribute digital books via a simple catalog format. The mechanism through which compatible Reading Systems access the distributed catalog has three components: eBook content, XML catalog metadata, and an HTTP transport for the catalog. OPDS-compatible Reading Systems must support IDPF EPUB, and may optionally support additional formats.