We got stuck on a strange issue recently when trying to upload a file to a server using the Flex FileReference class. The file data was sent to a PHP script that processed the file, and then returned some information about the file as a response. What we found was that it worked fine on a PC, but on a MAC when Flex received the response in the uploadCompleteData handler the returned data was empty! Turns out extra white space after the php tags in a php include caused the issue.