How to implement Facebook app "Like before use"?

Many Facebook app 开发者_开发技巧ask you "like" it before use it. How to implement it? Is there special API for it?

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FBML pages have been deprecated and you can now only create iframe fan pages. When the user navigates to your page, Facebook sends a signed_request parameter that you will need to decode. This article has a walkthrough on how to do it.

function parsePageSignedRequest() {
    if (isset($_REQUEST['signed_request'])) {
      $encoded_sig = null;
      $payload = null;
      list($encoded_sig, $payload) = explode('.', $_REQUEST['signed_request'], 2);
      $sig = base64_decode(strtr($encoded_sig, '-_', '+/'));
      $data = json_decode(base64_decode(strtr($payload, '-_', '+/'), true));
      return $data;
    }
    return false;
}

if($signed_request = parsePageSignedRequest()) {
    if($signed_request->page->liked) {
      echo "This content is for Fans only!";
    } else {
      echo "Please click on the Like button to view this tab!";
    }
  }

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If your app is an iframe loaded in a Page tab signed_request can be used. http://developers.facebook.com/docs/authentication/signed_request/