Get driving distance between two points using Google Maps API

Question

Using Google Maps API, I'm attempting to get the driving distance between two points. I now have a function that calculates direct distance:

This function get lat and lng:

function get_coordinates($city, $street, $province)
{
    $address = urlencode($city.','.$street.','.$province);
    $url = "http://maps.google.com/maps/api/geocode/json?address=$address&sensor=false&region=Poland";
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_PROXYPORT, 3128);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
    $response = curl_exec($ch);
    curl_close($ch);
    $response_a = json_decode($response);
    $return = array('lat' => $response_a->results[0]->geometry->location->lat, 'long' => $long = $response_a->results[0]->geometry->location->lng);

    return $return;
}

This function calculates distance between two points based on lat & lng:

function getDistanceBetweenPoints($lat1, $lon1, $lat2, $lon2) {
    $theta = $lon1 - $lon2;
    $miles = (sin(deg2rad($lat1)) * sin(deg2rad($lat2))) + (cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta)));
    $miles = acos($miles);
    $miles = rad2deg($miles);
    $miles = $miles * 60 * 1.1515;
    $kilometers = $miles * 1.609344;
    return $kilometers;
}

This function prints distance by kilometers:

function get_distance($lat1, $lat2, $long1, $long2)
{
    /* These are two points in New York City */
    $point1 = array('lat' => $lat1, 'long' => $long1);
    $point2 = array('lat' => $lat2, 'long' => $long2);

    $distance = getDistanceBetweenPoints($point1['lat'], $point1['long'], $point2['lat'], $point2['long']);
    return $distance;
}

Usage:

$coordinates1 = get_coordinates('Katowice', 'Korfantego', 'Katowicki');
$coordinates2 = get_coordinates('Tychy', 'Jana Pawła II', 'Tyski');

echo 'Distance: <b>'.round(get_distance($coordinates1['lat'], $coordinates2['lat'], $coordinates1['long'], $coordinates2['long']), 1).'</b> km';

However, this code obtains a direct distance. I require a travel distance. How can I use the Google Maps API to get the driving distance?