I'm trying to get all records that belongs to last month, so far I managed to get all from last month but to date today, I'm not sure how I can get only for last month
$revenueMonth = Callback::where('created_at', '>=', Carbon::today()->startOfMonth()->subMonth())->sum('payment');
More clear solution for your problem:
$revenueMonth = Callback::whereMonth(
'created_at', '=', Carbon::now()->subMonth()->month
);