calculate interent after 13 days code example

Example: calculate 1 day interest

#How to calculate 1 day Interest by php 
<?php 
  $amt = 2000;
  $int = 12;
  $days = 30;
  $months= 12;
  $calc = ($amt * $int/100)/ $months / $days;
  echo $calc

#output = 0.66
?>