Check if user likes page via Graph API

Question

I have seen a few similar questions but I think this is new...

I am trying to add a popover on a blog that will contain a Facebook like button for a Facebook page an

Answer 1

You can tell if a user likes a page from the signed_request which is sent in the request from facebook. You don't have to query the graph api.

We use c# and get it like so:

protected void Page_Load(object sender, EventArgs e)
    {
        Result = DecodePayload(Request["signed_request"]);
    }

    public JObject DecodePayload(string payload)
    {
        var encoding = new UTF8Encoding();
        var decodedJson = payload.Split('.')[1].Replace("=", string.Empty).Replace('-', '+').Replace('_', '/');
        var base64JsonArray = Convert.FromBase64String(decodedJson.PadRight(decodedJson.Length + (4 - decodedJson.Length % 4) % 4, '='));
        Json = encoding.GetString(base64JsonArray);
        var result = JObject.Parse(Json);
        return result;
    }

Then in the page

<% if (Result["page"] == null || Result["page"]["liked"] == null || !(bool) Result["page"]["liked"])
     {%> 
 Content if liked
<%}%>

One more important thing as of the 30th March the page layout is changing to timeline and you should be aware of a bug currently that does not refresh the page upon liking it see this:

Does anyone know a fix for the lack of page refresh on facebook timeline pages when liking?

UPDATE

The php for decoding the signed request is:

   function parse_signed_request($signed_request, $secret) {
  list($encoded_sig, $payload) = explode('.', $signed_request, 2); 

  // decode the data
  $sig = base64_url_decode($encoded_sig);
  $data = json_decode(base64_url_decode($payload), true);

  if (strtoupper($data['algorithm']) !== 'HMAC-SHA256') {
    error_log('Unknown algorithm. Expected HMAC-SHA256');
    return null;
  }

  // check sig
  $expected_sig = hash_hmac('sha256', $payload, $secret, $raw = true);
  if ($sig !== $expected_sig) {
    error_log('Bad Signed JSON signature!');
    return null;
  }

  return $data;
}

function base64_url_decode($input) {
  return base64_decode(strtr($input, '-_', '+/'));
}