Notice: A non well formed numeric value encountered in C:\wamp64

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$datetime1 = new DateTime($orderdate);
                        $datetime2 = new DateTime($currdate);

                        $interval = date_diff($datetime1, $datetime2);
                        $interval->format('%m months');
                        $months_diff = $interval->format('%m months');

                        for($i = 1; $i <=($months_diff+1); $i++){
                            $totalFees = $fees + $totalFees;