PHP Universal Feed Generator is a good answer. I had just to modify the code accordingly to a comment on the webpage to handle correcly french characters.
http://www.ajaxray.com/blog/2008/03/08/php-universal-feed-generator-supports-rss-10-rss-20-and-atom/#comment-341
Like Kereste says (1. June) FeedWrite
makes problems with some none-english
utf-8 characters like ä etc.. this is
because xml only knows 5 entities
(&,”,’,). So I changed line 298 in the
source code into
$nodeText .= (in_array($tagName,
$this->CDATAEncoding))? $tagContent :
str_replace ( array ( ‘&’, ‘”‘, “‘”, ”
), array ( ‘&’ , ‘"’, ‘'’ , ‘<’ , ‘>’
), $tagContent);
to avoiod converting charakters into
wrong xml entities.