error FacebookResponseException when use SDK PHP to post a link - php

Question

i'm using sdk php to post a link. here is my function (https://developers.facebook.com/docs/php/howto/postwithgraphapi/)
public function shareFB(Request $request, $slugify)
{
$petition = Petitions::where('slug', $slugify)->first();
// $access_token = auth()->user()->social_account->access_token;
$fb = new Facebook([
'app_id' => config("services.facebook.client_id"),
'app_secret' => config("services.facebook.client_secret"),
'default_graph_version' => 'v2.2',
]);
$helper = $fb->getRedirectLoginHelper();
$accessToken = $helper->getAccessToken();
$linkData = [
'link' => route('petitions.index',$petition->slug),
'message' => $request->get('comment-fb'),
];
try {
$response = $fb->post('/me/feed', $linkData, '$access_token');
} catch(Facebook\Exceptions\FacebookResponseException $e) {
echo 'Graph returned an error: ' . $e->getMessage();
exit;
} catch(Facebook\Exceptions\FacebookSDKException $e) {
echo 'Facebook SDK returned an error: ' . $e->getMessage();
exit;
}
$share = $response->getGraphNode();
return redirect()->route('petitions.index',$petition->slug);
}
and error is Invalid OAuth access token, maybe i get wrong access token. Anyone help me solve that/ Thank u