Diferencia entre dias PHP

<?php
  
  // Function to find the difference 
  // between two dates.
  function dateDiffInDays($date1, $date2) 
  {
      // Calculating the difference in timestamps
      $diff = strtotime($date2) - strtotime($date1);
  
      // 1 day = 24 hours
      // 24 * 60 * 60 = 86400 seconds
      return abs(round($diff / 86400));
  }
  
  // Start date
  $date1 = "17-09-2018";
  
  // End date
  $date2 = "31-09-2018";
  
  // Function call to find date difference
  $dateDiff = dateDiffInDays($date1, $date2);
  
  // Display the result
  printf("Difference between two dates: "
     . $dateDiff . " Days ");
?>