Populating Other People's Forms

I would like to write a script that would populate someone else's form on the net and get the results. Off hand, I imagine I would use fopen() to open up the web page and pass some variables to it. The problem I have is that the form I want to populate is a multi-step CGI. Is this possible? How do I continue to pass variables to a second or third step, sending variables passed on what the form says to do next (which is predictable)? This is considering I do not have the cooperation of the web site I am trying to fill out the form for step by step, and I would like to write this so it is done in a behind-the-scenes, automated way. For example: A customer registered with me could ask for a product. My 'agent' would go to the site where the product is sold, login, submit all of the appropriate information for purchase and shipment, and then come back with (hopefully) a success message (along with any pertinent information from the purchase). Anyone have any ideas, suggestions, or can point me in the right direction? Thanks, -Mike