The xml-declaration must be on the absolute first line in the output. I.e. no blank lines or spaces before the xml-declaration tag.
This is valid:
<?xml version="1.0" encoding="UTF-8" ?>
This is not:
<?xml version="1.0" encoding="UTF-8" ?>
Check whether <?xml version="1.0" encoding="utf-8"?> is the first line in the feed file. No empty lines or spaces!
If PHP is spitting out any notices/warnings or such, those will malform the feed. Try setting error_reporting to zero before the feed is sent to test:
error_reporting(0);
good rule of thumb when using php class files and such, never ?> your class files. Only use ?> in template-type files where you are going to have regular output afterwards. All of the major packages do this now for exactly the above reasoning.