Unknown Entities and Atom/Rss-Feeds: HowTo?

[S.T]

Hi!

I've spend this weekend trying and coding a php-based atom- and
rss-feed writer. I don't know if this is the right place for my
questions. However, since both feeds are based on xml, maybe someone
can help.

I understand that I have to replace all unknown entities, such as
"ä". I wonder what's the common way to do so. For XML-Documents,
I'd prefer to make use of an external DTD, say, referring to
http://www.w3.org/TR/REC-html40/sgml...es.html#h-24.2 Having
looked up some typical atom feed on the web, I realized that they DO
NOT include external DTD, and still, they work fine with characters
that are being declared as "unknown entities" in my documents. E.g.
typical german entities in opera's atom feed:
http://my.opera.com/sas1010/xml/atom/album/104384/ actually DO work,
but mine don't :-(((((

I wondered if I am to replace all entities by they corresponding codes
( Ä by Ä ) using my php-code, but this cannot be the
appropiate way, can it?

So what is your experience? Any suggestions?

Thanks

S.T.