I have an ISAPI filter that I am using to do URL rewriting for my CMS. I am processing SF_NOTIFY_PREPROC_HEADERS notifications, and trying to do this:

DWORD ProcessHeader(HTTP_FILTER_CONTEXT *con, HTTP_FILTER_PREPROC_HEADERS *head)
{
    head->SetHeader(con, "test1", "aaa");
    con->AddResponseHeaders(con, "test2:bbb\r\n", 0);
    return SF_STATUS_REQ_NEXT_NOTIFICATION;
}

However, I can't seem to read these values using server variables or response headers in classic ASP or PHP. The values are missing. I'm expecting either my "test1" or "test2" header values to appear, but they are not. Am I doing something wrong here?