Laravel insert date created_at from 1 to 30

I try to insert data in my custom table

$month =  date('m');
        $year =  date('y');
        $number = cal_days_in_month(CAL_GREGORIAN, $month, $year);

        for ($i = 0; $i < $number; $i++){
            $test= new Test();
            $test->cuid = $user->id;
            $test->name = $user->name;
            $test->month = date('m');
            $test->created_at = '2018-04-'+$i;
            $test->save();
        }

i try to insert number record in a month

let's say April have 30 days i try to insert it working fine but in created_at field i need insert 2018-04-01 to 2018-04-30 but this is insert only current date so that i try above but not working

via Chebli Mohamed