Find distance between two cities using lattitude and longitude using Google Maps

Distance can be calculated using lattitude and longitude obtained from google maps api, using the following code:
//Get the lattitude and longitude of two cities first -- returns json format data
$address1 = urlencode($addressLine1).",+".urlencode($city1).",+".urlencode($statePrefix1);
$address2 = urlencode($addressLine2).",+".urlencode($city2).",+".urlencode($statePrefix2);

$geoCode1 = file_get_contents("http://maps.google.com/maps/api/geocode/json?address=".$address1."&sensor=false");
                    $geoCode2 = file_get_contents("http://maps.google.com/maps/api/geocode/json?address=".$address2."&sensor=false");

//now get lat and long
$output1 = json_decode($geoCode1);
$lat1 = $output1->results[0]->geometry->location->lat;
$lng1 = $output1->results[0]->geometry->location->lng;

$output2 = json_decode($geoCode2);
$lat2 = $output2->results[0]->geometry->location->lat;
$lng2 = $output2->results[0]->geometry->location->lng;

//now get the distance
calculate_distance($lat1, $lng1, $lat2, $lng2, "K");

//function calculate_distance
function calculate_distance($lat1, $long1, $lat2, $long2, $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;
        }
    }