Facebook’s Real-time Updates documentation is not very explicit on how to subscribe to a feed, especially when all I am looking for is an example. So after figuring out what OAuthException (#15) "This method must be called with an app access_token" was trying to tell me and fixing the problem, I figured I would share my PHP example here. Hopefully, it will be helpful to you.* There are two parts that you need to set up:

1. Setup an endpoint for the subscription
2. Post to Facebook and tell them you are subscribing
1. Get an access_token
2. POST to Facebook with cURL

1. Setup an Endpoint

This part is actually pretty easy because Facebook actually gives you example code (yay!). Put that file on a web server somewhere with an external ip so that Facebook can reach it. Replace 'abc' in define('VERIFY_TOKEN', 'abc');. Save this value for later.

2. Post to Facebook

Now comes the part that is not well documented. In order to subscribe you need to first specifically get an access_token, then POST to Facebook.

A. Get an access_token

$app_id = APP_ID;
$secret = APPLICATION_SECRET;
$token_url =    "https://graph.facebook.com/oauth/access_token?" .
        "client_id=" . $app_id .
        "&client_secret=" . $secret .
        "&grant_type=client_credentials";
$response = @file_get_contents($token_url);
$params = null;
parse_str($response, $params);

B. POST to Facebook with cURL

Remember when we defined the VERIFY_TOKEN earlier? Replace VERIFY_TOKEN with that value in this code, and ENDPOINT_URLwith the url of the file from part 1.

$base_url = "https://graph.facebook.com/$app_id/subscriptions";
$url = $base_url . "?access_token=" . $params['access_token'];
$fields_string = '';
$fields = array(
            'callback_url=ENDPOINT_URL',
            'object=user',
            'verify_token=VERIFY_TOKEN',
            'fields=feed'
        );
$fields_string = implode("&", $fields);
//open connection
$ch = curl_init();
//set the url, number of POST vars, POST data
curl_setopt($ch,CURLOPT_URL,$url);
curl_setopt($ch,CURLOPT_POST,count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS,$fields_string);
//execute post
$result = curl_exec($ch);
//close connection
curl_close($ch);

Once you visit this page successfully, Facebook will call the callback_url defined, verify that the VERIFY_TOKEN in both parts match, and then start sending Real-time Updates to your endpoint.

That was easy.

*By the time you find this post and need help, you can either learn from my example code or, more likely, bang your head against a wall once you realize Facebook has changed its API once again.