Calculate the distance between two latitude & longitude

Calculate the distance between two latitude & longitude

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;

     }

}