Simple Function to add to Facebook's facebook-php-sdk to grab an extended token for a user who has been authorized. Should be used when extending the BaseFacebook Class

extendAccessToken.php

<?php 
function extendAccessToken() {
    
    try {
      // need to circumvent json_decode by calling _oauthRequest
      // directly, since response isn't JSON format.
      $access_token_response =
        $this->_oauthRequest(
          $this->getUrl('graph', '/oauth/access_token'),
          $params = array('client_id' => $this->getAppId(),
                          'client_secret' => $this->getAppSecret(),
                          'grant_type' => 'fb_exchange_token',
                          'fb_exchange_token' => $this->getAccessToken()));
    } catch (FacebookApiException $e) {
      // most likely that user very recently revoked authorization.
      // In any event, we don't have an access token, so say so.
      return false;
    }

    if (empty($access_token_response)) {
      return false;
    }

    $response_params = array();
    parse_str($access_token_response, $response_params);
    if (!isset($response_params['access_token'])) {
      return false;
    }
    return $response_params['access_token'];
  }