how do I override php://input when doing unit tests

I'm trying to write a unit test for a controller using Zend and PHPUnit

In the code I get data from php://input

$req = new Zend_Controller_Request_Http();
$data = $req->getRawBody();

My code works fine when I test the real application, but unless I can supply data as a raw http post, $data will always be blank. The getRawBody() method basically calls file_get_contents('php://input'), but how do I override this in order to supply the test data to my application.