How to put a progress bar into wordpress page while executing another php file

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PHP

e-commerce

PayPal

Hi all,

Let me explain my question.

I am working on an e-commerce project. What happens is that after a buyer successfully makes payment through paypal, he gets redirected back to a Transaction page providing him with his purchase details.

This Transaction page is part of a wordpress plugin, and is in php.
What I'm trying to do then, is start another php process from another file, if the payment is successful. (This process takes ~16 secs)

So far, I am able to do so by calling this process via a function call.
This however, will leave the user with a blank screen until only after the whole 16s process is finished. Therefore it seems obvious that what I need here is to be able to put in a progress bar. But I'm at a loss on how to do it.

Creating a 1 or 2 webpage progress bar with ajax I have done before, but this time its
a wordpress plugin php functions file. I also cannot output contents to the browser, since it will protest that the Header information has already been sent. How am I gonna do this?

I'm using the WP-ecommerce plugin by the way.

Thanks,
Brian