Graph returned an error: Invalid appsecret_proof provided in the API argument

i am using facebook PHP sdk (v4) to fetch user information, after installing SDK, i add the code

$fb = new FacebookFacebook([
  'app_id' => 'my app id',
  'app_secret' => 'my app secret',
  'default_graph_version' => 'v2.5',
]);

try {
  // Returns a `FacebookFacebookResponse` object
 $access_token= //copied from my https://developers.facebook.com/tools/explorer/
  $response = $fb->get('/me?fields=id,name', '$access_token');
} catch(FacebookExceptionsFacebookResponseException $e) {
  echo 'Graph returned an error: ' . $e->getMessage();
  exit;
} catch(FacebookExceptionsFacebookSDKException $e) {
  echo 'Facebook SDK returned an error: ' . $e->getMessage();
  exit;
}

$user = $response->getGraphUser();

echo 'Name: ' . $user['name'];

but when i run the page it gives me this error,

Graph returned an error: Invalid appsecret_proof provided in the API argument

i am copying the app secret correctly, what could be the reason for such error and how should i fix this ?

Answer

You may want to read this: https://developers.facebook.com/docs/graph-api/securing-requests

appsecret_proof is a separate parameter that is generated by using the App Secret, but it is NOT the App Secret. Information about how to generate it can be found in the docs.