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;
      }
}

//USAGE

//Miles
echo distance(32.9697, -96.80322, 29.46786, -98.53506, "m") . " miles<br/>";

//Kilometers
echo distance(32.9697, -96.80322, 29.46786, -98.53506, "k") . " kilometers<br/>";

//Nautical miles
echo distance(32.9697, -96.80322, 29.46786, -98.53506, "n") . " nautical miles<br/>";

http://www.phpsnippets.info/calculate-distances-in-php

Calcular distancias entre puntos geográficos

PHP