$order = 'asc'; // 'asc' or 'desc'

foreach($turnovers as $i => $turnover) {
    // .....
    $currentDate = new DateTime($turnover->turnoverDate);
    $currentMonth = $currentDate->format('m');

    $nextDate = $turnover;
    // check if last item in array
    if ($i < count($turnovers) - 1 && $order === 'asc') {
        $nextDate = $turnovers[$i+1];
    } else if ($i > 0 && $order === 'desc') {
        $nextDate = $turnovers[$i-1];
    }

    $nextDate = new DateTime($nextDate->turnoverDate);
    $nextMonth = $nextDate->format('m');

    if($currentMonth != $nextMonth) {
        $print .= "<tr class=\"table-success\">";
    }
    else {
        $print .= "<tr>";
    }
    // ....
}