Facebook PHP SDK: Cannot get Access Token from JavascriptHelper Class - php

Question

I have am using FB's Javascript SDK on the client and PHP SDK on the server.
I can see when I use getSignedRequest() I get the signed request containing the code property and the correct userId. However for the life in me I can't figure out how to go on to use this signedRequest to obtain a valid access token which I can use to create further calls for things like user information. I have tried querying for the access code by attempting to instantiate a signed request or calling the ->getAccessToken() method on the $facebook object as well as the jsHelper object but nothing is returned to me. Here's my code:
require_once 'src/autoload.php';
$facebook = new Facebook\Facebook([
'app_id' => $AppId,
'app_secret' => $AppSecret,
'default_graph_version' => 'v2.8',
]);
$jsHelper = $facebook->getJavaScriptHelper();
$signedRequest = $jsHelper->getSignedRequest();
if ($signedRequest) {
try {
$codeField = $signedRequest->get('code');
$g = $facebook->getAccessToken();
//I would hope this would be the access token, but sadly it's an empty
value.
error_log($g);
} catch(Facebook\Exceptions\FacebookResponseException $e) {
error_log("fbResp Ex: ". $e->getMessage());
exit;
} catch(Facebook\Exceptions\FacebookSDKException $e) {
error_log("SDK Ex: ". $e->getMessage());
exit;
}

Answer 1

I never used JavaScriptHelper but AccessToken comes from helper not Facebook object itself. Try
$g = $jsHelper->getAccessToken();