Have you tried using an HTTP sniffer to capture the traffic of the request and response when submitting to the php URL? Perhaps you're getting back an HTML-formatted error page instead of XML?
Do you really need the result of the post to the PHP page decoded into AS3 objects? If not, try changing the "addArtist" HTTPService to just leave the result as plain text by specifying the resultFormat attribute with a value of "text". Are you sure the PHP URL is correct? Can you try a fully qualified URL instead of the relative one?
