Calculate Distance Between Two ZIP Codes or Postal Codes

<?php

/*::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::                                                                         :*/
/*::  This routine calculates the distance between two points (given the     :*/
/*::  latitude/longitude of those points). It is being used to calculate     :*/
/*::  the distance between two zip codes or postal codes using our           :*/
/*::  Zipcodeworld(tm) and Postalcodeworld(tm) products.                     :*/
/*::                                                                         :*/
/*::  Definitions:                                                           :*/
/*::    south latitudes are negative, east longitudes are positive           :*/
/*::                                                                         :*/
/*::  Passed to function:                                                    :*/
/*::    lat1, lon1 = latitude and longitude of point 1 (in decimal degrees)  :*/
/*::    lat2, lon2 = latitude and longitude of point 2 (in decimal degrees)  :*/
/*::    unit = the unit you desire for results                               :*/
/*::           where: 'm' is statute miles                                   :*/
/*::                  'k' is kilometers (default)                            :*/
/*::                  'n' is nautical miles                                  :*/
/*::  United States zip code and Canadian postal code databases with latitude & :*/
/*::  longitude are available at http://www.zipcodeworld.com                 :*/
/*::                                                                         :*/
/*::  For enquiries, please contact [email protected]                   :*/
/*::                                                                         :*/
/*::  Official web site: http://www.zipcodeworld.com                         :*/
/*::                                                                         :*/
/*::  Hexa Software Development Center all rights reserved 2004            :*/
/*::                                                                         :*/
/*::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
function distance($lat1, $lon1, $lat2, $lon2, $unit) { 

  $theta = $lon1 - $lon2; 
  $dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) +  cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta)); 
  $dist = acos($dist); 
  $dist = rad2deg($dist); 
  $miles = $dist * 60 * 1.1515;
  $unit = strtoupper($unit);

  if ($unit == "K") {
    return ($miles * 1.609344); 
  } else if ($unit == "N") {
      return ($miles * 0.8684);
    } else {
        return $miles;
      }
}

echo distance(32.9697, -96.80322, 29.46786, -98.53506, "m") . " miles<br>";
echo distance(32.9697, -96.80322, 29.46786, -98.53506, "k") . " kilometers<br>";
echo distance(32.9697, -96.80322, 29.46786, -98.53506, "n") . " nautical miles<br>";

?>