If you are loading into a level, might I purswade you to load into an MC. I find that it has far less complications and is alot easier to work with. The _level102 thing is a little high up there, I am a little curious about that one too. If you use an MC then you will not have to worry about conflicting levels.
one more thing.....like mock said....you need to use the absolute path to reference to your php file if it is on a different server from your swf...however this should not work. Macromedia has the security feature which won't let you do this. You php needs to be no the same server as your swf but your txt can be on a different server if you use php (like in the GB) to read and write to it. You need to use the absolute, not relative, path to your textfile in the php if the two are on different servers.....so use http://yourdomain.com/GuestBook.txt....and
make sure that you are setting the chmod (permissions)m of your textfile to 666 or 777. 666 is better for security purposes. Keep posting your progress. Lates.