wow, i can't believe I've never run into this before, I guess I never had to send xml TO a php script from flash before. Anyway, there are two ways to read the xml being sent to the php page. The first is $HTTP_RAW_POST_DATA. This is a global variable created to hold content sent to the page that who's content type is not set to application/x-www.form-urlencoded or multipart/form-data. Using $HTTP_RAW_POST_DATA can be a little flaky because it depends on version/php ini information.
Another way to access the raw php data being sent to the page is to read from the input stream directly by using php://input. You would use that any place where you would pass a file name, such as:
According to the php docs, using php://input uses less momory than $HTTP_RAW_POST_DATA.
I've also noticed that sometimes only one or the other will work on some server setups. Unfortunately, I never discovered the cause.