How can I integrate one of those "refer a friend" boxes into my facebook page and reward people for using it?

So I have a client who wants to integrate something into his facebook page that will basically give a coupon to people for sharing the page with 3 or more friends.

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Ideally I'd like an interface similar to the way wildfire app does it with contests ala http://apps.facebook.com/contestshq/ Here is a screenshot:

Basically I want this select your friends module to lead to a "thanks you did it, here's your coupon code: XXX" page. Also, these coupons need to be unique so I can't just land the users on a static page it needs to somehow play with my PHP code.

Any high level ideas on how to approach would be great. I have fooled around a little with the Connect API but I'm not sure if that would be overkill or not.

UPDATE: Didn't really mention it but this should all take place in the facebook page [possibly in an iframe] without the end user leaving facebook.com

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Rewarding users for the act of sharing a link like that is a violation of Facebook Policy, Section IV.1

You must not incentivize users to use (or gate content behind the use of) Facebook social channels, or imply that an incentive is directly tied to the use of our channels

The Wildfire example detaches any incentive from the mere act of sharing, they just offer it as an option, not a requirement.

Now, you could have a workflow of -

1. User clicks Like to enter app (commonly called likegating, not great UEX though)
2. Interacts with app
3. 'Would you like to share this great app/offer/whatever with your friends?' Yes / No (clearly stated that it's not a requirement).
4. User gets coupon regardless of actions in step 2.