No browser is going to let you do that fortunately. I say fortunately because the browser has no way of knowing that site A and B have a two way relationship so as an internet user I am grateful of this protection.
Your only viable option is to do something on the server end. Maybe write out XML in the parent JSP app, then post that url to the PHP for parsing. Otherwise I know java can access HTTP protocol directly. You could probably write a little java that submits the data you need directly into the PHP server via a GET.
Sorry