PHP Facebook SDK 4不会提示用户获取权限

When users are trying to log in to my app the dialog box, asking for permission is not apearing. My code is:

           $facebook = new Facebook(array(
           'appId' => 'myappid',
           'secret' => 'mysecret'
            ));

        if($facebook->getUser() == 0){
        $loginUrl = $facebook->getLoginUrl(array(
       'scope' => 'email'
        ));

       else{
           $api = $facebook->api('me');
           echo "Hi " . $api[name];
           $user_profile = $facebook->api('me', array('fields' => 'email'));
           .....
       } 

I have also tried to use this:

  $loginUrl = $facebook->getLoginUrl(array(
 'req_perms' => 'publish_stream, read_friendlists'
  ));

But again users are not asked for permissions to allow it. I guess that this is probably due to some settings configuration...