如何创建公共facebookRequest类？ php facebook SDK

I would like to get the city graph by facebook php sdk:

public static function getPublicGraphObject($Id)
{
  $request = new FacebookRequest(null, 'GET', '/'.$Id);
  $response = $request->execute();
  $graphObject = $response->getGraphObject();

  return $response->getGraphObject()->asArray();
} 

because the city is public data, there is no need for session ? unfortunately I get this error Facebook\FacebookRequest::__construct() must be an instance of Facebook\FacebookSession, null given ? what should I do ?

To initialize the FacebookRequest class, you need to pass it a FacebookSession instance as first parameter, you can not simply pass NULL instead.

If you do not have a current user logged in to your app (and therefor no user session), or want to make a request not on behalf of the current user, but simply “as” your app instead, create a new app session and pass that – the FacebookSession class has a newAppSession method for that purpose.