$HTTP_RAW_POST_DATA and preferred: php://stdin already
exist for this.

A comment from http://www.php.net/variables.external:

"For what I understand, since PHP 4.3 it is possible to access the content of a POST request (or other methods as well) as an input stream named php://input, example:

readfile("php://input");   
[to display it]

or

$fp = fopen("php://input", "r");   
[to open it and then do whatever you want]

This is very useful to access the content of POST requests which actually have a content (and not just variable-value couples, which appear in $_POST).

This substitutes the old $HTTP_RAW_POST_DATA variable available in some of the previous 4.x versions. It is available for other upload methods different from POST too, but it is not available for POSTs with multipart/form-data content type, since the file upload handler has already taken care of the content in that case."

This is bogus, as sniper has said, $HTTP_RAW_POST_DATA and php://input exist for this purpose.