Exchanging Facebook Auth Code for Access Token using the PHP SDK - facebook

Question

I am trying to build a server-to-server auth flow using the Facebook PHP SDK and no Javascript, as outlined here. So far, I have successfully created a LoginUrl that lets the User sign in with Facebook, then redirect back to my App and check the state parameter for CSFR protection.
My Problem is, that I can't seem to get the API-call working that should swap my Auth Code for an access token. I pillaged every similar problem anyone else that Google was able to find had encountered for possible solutions.
Yet the end result was always the same: no access token, no error message that I could evaluate.
Researching the topic yielded the following advice, which I tested:
The URL specified in the App Settings must be a parent folder of $appUrl.
use curl to make the request instead of the SDK function api()
I've been at this for 2 days straight now and really could use some help.
<?php
require '../inc/php-sdk/src/facebook.php';
// Setting some config vars
$appId = 'MY_APP_ID';
$secret = 'MY_APP_SECRET';
$appUrl = 'https://MY_DOMAIN/appFolder';
$fbconfig = array('appId'=>$appId, 'secret'=>$secret);
$facebook = new Facebook($fbconfig);
// Log User in with Facebook and come back with Auth Code if not yet done
if(!(isset($_SESSION['login']))){
$_SESSION['login']=1;
header('Location: '.$facebook->getLoginUrl());
}
// process Callback from Facebook User Login
if($_SESSION['login']===1) {
/* CSFR Protection: getLoginUrl() generates a state string and stores it
in "$_SESSION['fb_'.$fbconfig['appId'].'_state']". This checks if it matches the state
obtained via $_GET['state']*/
if (isset($_SESSION['fb_'.$fbconfig['appId'].'_state'])&&isset($_GET['state'])){
// Good Case
if ($_SESSION['fb_'.$fbconfig['appId'].'_state']===$_GET['state']) {
$_SESSION['login']=2;
}
else {
unset($_SESSION['login']);
echo 'You may be a victim of CSFR Attacks. Try logging in again.';
}
}
}
// State check O.K., swap Code for Token now
if($_SESSION['login']===2) {
$path = '/oauth/access_token';
$api_params = array (
'client_id'=>$appId,
'redirect_uri'=>$appUrl,
'client_secret'=>$secret,
'code'=>$_GET['code']
);
$access_token = $facebook->api($path, 'GET', $api_params);
var_dump($access_token);
}

Answer 1

The easiest way I found to do this is to extend the Facebook class and expose the protected getAccessTokenFromCode() method:
<?php
class MyFacebook extends Facebook {
/** If you simply want to get the token, use this method */
public function getAccessTokenFromCode($code, $redirectUri = null)
{
return parent::getAccessTokenFromCode($code, $redirectUri);
}
/** If you would like to get and set (and extend), use this method instead */
public function setAccessTokenFromCode($code)
{
$token = parent::getAccessTokenFromCode($code);
if (empty($token)) {
return false;
}
$this->setAccessToken($token);
if (!$this->setExtendedAccessToken()) {
return false;
}
return $this->getAccessToken();
}
}
I also included a variation on the convenience method I use to set the access token, since I don't actually need a public "get" method in my own code.