problem between 28 and 29 February, please change date and see result
$cmncdate = '2019-02-29';
$time=strtotime($cmncdate);
$day=date("d",$time);
$mnth=date("m",$time);
if ($day>=1 AND $day<11)
{ $nday = '10'; $newDate = date("Y-m-$nday", strtotime($cmncdate));
echo $newDate; }
elseif ($day>10 AND $day<21)
{ $nday = '20'; $newDate = date("Y-m-$nday", strtotime($cmncdate));
echo $newDate; }
elseif ($day>20 AND $day<31 AND $mnth!=2)
{ $nday = '30'; $newDate = date("Y-m-$nday", strtotime($cmncdate));
echo $newDate; }
elseif ($day>20 AND $day<29 AND $mnth=2)
{ $nday = '28'; $newDate = date("Y-m-$nday", strtotime($cmncdate));
echo $newDate; }
elseif ($day=29 AND $mnth=2)
{ $nday = '29'; $newDate = date("Y-m-$nday", strtotime($cmncdate));
echo $newDate; }