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)

Easy to Use
As simple as:
require('rss_fetch.inc');
$rss = fetch_rss($url);