Use a library.

An update for anyone that may stumble upon this thread:

A very similar question was asked in The best PHP lib/class to generate RSS/Atom and it lead to a number of good lib/roll your own recommendations.

Wikipedia has an example of what an ATOM feed looks like. Feel free to modify this very basic RSS class that I wrote a long while ago to create a very simple RSS feed:

class RSSFeed
{       
    var $feedHeader;
    var $feedItems;

    /* Class Constructor */
    function RSSFeed()
    {
        //do some contruction
        $this->feedHeader =   ;
        $this->feedItems =   ;
    }

    function setFeedHeader($title, $link, $description, $copyright, $lastBuildDate, $ttl)
    {
        $this->feedHeader =  <?xml version="1.0" encoding="UTF-8"?><rss version="2.0"><channel> ;
        $this->feedHeader .=  <title> .$title. </title> ;
        $this->feedHeader .=  <link> .$link. </link> ;
        $this->feedHeader .=  <description> .$description. </description><copyright> .$copyright. </copyright> ;
        $this->feedHeader .=  <language>en-GB</language><lastBuildDate> .$lastBuildDate.  GMT</lastBuildDate><ttl> .$ttl. </ttl> ;
    }

    function pushItem($title, $link, $description, $pubDateTime)
    {
        $item =  <item><title>  . htmlentities(stripslashes($title)) .  </title> ;
        $item .=  <link>  . $link .  </link> ;
        $item .=  <guid>  . $link .  </guid> ;
        $item .=  <description>  . htmlentities(stripslashes($description)) .  </description> ;

        $item .=  <pubDate>  . $pubDateTime .   GMT</pubDate></item> ;

        $this->feedItems = $item . $this->feedItems;
    }

    function writeOutFeed($path)
    {
        $file = fopen($path, "w");
        fputs($file, $this->feedHeader);
        fputs($file, $this->feedItems);
        fputs($file,  </channel></rss> );
        fclose($file);
    }
}