如何计算laravel当月的总收入？

Question

  

现在我正在计算每年用户的总收入。但我想按月显示。

public function dashboard()
{
  $this->data['title'] = trans('backpack::base.dashboard');
  $customers = Tag::all();
  $total_earnings = 0;

  foreach ($customers as $customer) {
    if(Auth::User()->gym_code == $customer->gym_code)
      $total_earnings = $total_earnings + $customer->amount; 
  } 

  // set the page title
  $this->data['earnings'] = $total_earnings;
  return view('backpack::dashboard', $this->data);
}