how to read facebook signed_request to get user_id

According to Facebook - Authentication within a Canvas Page Document, they say that we will be getting a signed_request which consists a JSON object. Now they say that signed_request can be get through $_POST['signed_request'] I agree its working for me.

Now according to them if the user is logged in i will be getting a JSON object value like this:-

{
  "expires":UNIXTIME_WHEN_ACCESS_TOKEN_EXPIRES,
  "algorithm":"HMAC-SHA256",
  "issued_at":UNIXTIME_WHEN_REQUEST_WAS_ISSUED,
  "oauth_token":"USER_ACCESS_TOKEN",
  "user_id":"USER_ID",
  "user":{
    "country":"ISO_COUNTRY_CODE",
    "locale":"ISO_LOCALE_CODE",
    ...
  }
}

Now i want to fetch the user_id out of this so i am using this piece of code but its not working:-

if(isset($_POST['signed_request']))
{
    echo 'YES';
    $json = $_POST['signed_request'];
    $obj = json_decode($json);
    print $obj->{'user_id'};    
}

It just print the YES. Why is it so?

I have read somewhere that without app authentication i will not be able to extract the user_id but according to the facebook, this is the 1st step and authenticating the application would be 4th. I am new to it, if somebody can please help me, it will be of great help. Thanks.

I think it failed at json_decode($json) because $json is not a valid json string, as you've mentioned in comment about print_r($_POST['signed_request']);.

According to Facebook - Authentication within a Canvas Page Document, the signed_request parameter is encoded and, parsing the signed_request string will yield a JSON object.

if you're using the PHP SDK, just as Abhishek said in the comment, $facebook->getSignedRequest(); will give you the decoded json.

look here for more details on the Signed Request