Returning entire POST

Using PHP on an Apache server, how do I capture and forward the entire contents of a POST exactly as it came in. Those familiar with paypal know that they offer the "Instant Payment Notification" where the paypal server will submit (to a predetermined page on your server) a POST containing all the variables and values of the paypal transaction. Instant Payment Notification then requires the page to return the entire POST exactly as it is received with one extra key/value on the end of it (for validation of information contained). What I'm looking for is something similar to the $QUERY_STRING or something to that nature. I can break apart the POST using $HTTP_POST_VARS and then re-assemble it as a string and then URL encode it, but I'm not guaranteed that it is EXACTLY like the original. Any ideas? Anyone ever used PHP for the PayPal Instant Payment Notification script? Thanks in advance, Kurt