haversineGreatCircleDistance.php

function haversineGreatCircleDistance($latitude1, $longitude1, $latitude2, $longitude2) {
      $earth_radius = 6371;
      $dLat = deg2rad($latitude2 - $latitude1);
      $dLon = deg2rad($longitude2 - $longitude1);
      $a = sin($dLat/2) * sin($dLat/2) + cos(deg2rad($latitude1)) * cos(deg2rad($latitude2)) * sin($dLon/2) * sin($dLon/2);
      $c = 2 * asin(sqrt($a));
      $d = $earth_radius * $c;
      return round($d * 0.621371,2);
  }