Forcing Visitors to Liking Facebook Page

I have a Facebook page.

We will start a giveaway. And we will give gifts to our Facebook Fans . They will give a promo code if开发者_如何学运维 like our page. BUt we want to sure, they are really fan of our fan page.

Because of this, I want to create a new tab which say "You have to like this page for get promotion code" to user . When visitor like our page, he / she can see content tab. If not, they can't see.

It was easy on static FBML, but know how can i do it with iFrame ? My tab is ready. Created a new app and then this app added to page.

you need to parse the signed_request parameter, it has a Boolean attribute called liked.

you may want to look at this question