How to access the values of the custom fields filled in at facebook registration form?

I was wondering how to access the custom fields people filled in after registering on your site with Facebook registration form.

I understand the usage of the PHP SDK environment a bit, and can access gender, name, etc. easily, but I have no idea how to do this with开发者_开发知识库 custom fields you yourself created.

I would prefer to receive an answer related to PHP SDK, but any is good.

thanks in advance!

Once the registration form is submitted, a signed_request holding ALL the data you need will be send back to your server on the URL you specify redirect_uri, how to "extract" these data is explained in the documentation (PHP Example reading signed_request section):

<?php
define('FACEBOOK_APP_ID', 'your_app_id');
define('FACEBOOK_SECRET', 'your_app_secret');

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, '-_', '+/'));
}

if ($_REQUEST) {
  echo '<p>signed_request contents:</p>';
  $response = parse_signed_request($_REQUEST['signed_request'], 
                                   FACEBOOK_SECRET);
  echo '<pre>';
  print_r($response);
  echo '</pre>';
} else {
  echo '$_REQUEST is empty';
}
?>