Automated Postback to alternate domain

Here's the scenario I could use some help with.

Visitor A comes to my site and clicks on a forum button verifying they were there. I then need to send a Postback to the sending site to let them know that Visitor A was in fact here on my site. I'm confused on how to make a PHP page generate and send a postback without any direct user intervention.


Site A: Page 1 has a Form with a Buttom
Site A: Page 2 Receives Post from Visitor click, processes as necessary, and then sends a notification $_POST to Site B
Site B: Receives $_POST data from Site A

Thanks for the help! Happy Holidays!

not sure about background posts think perhaps that'd have to be an ajax style request.

gets could be done via an fopen() or a simplexml request I guess

You need to send an asynchronous http post.
Its not easy/possible to do using the usual methods like CURL or fopen.

I did find this blog post about async posts. It looks like it works.

http://petewarden.typepad.com/searchbrowser/2008/06/how-to-post-an.html