I think what you are asking is how to get data from MySQL into an XML format that is defined by a DTD? I think there's not really a simple php-based way to do it. You'd have to write a php script that queries the MySQL database, and then formats the output in XML tags...?
Or are you asking if there is a way to have MySQL spit out a table & its contents into an XML format, and generate a DTD for it also?
Can you be more specific about what you're trying to do?