ACHTUNG. Auch bei OpenStreetMap gibt es Nutzungsbedingungen welche hier zu finden sind: OpenStreetMap API

function getOsmCoordinates($street, $zip = '', $city = '', $country = 'Germany')
{
  $address = $street . ', ' . $zip . ' ' . $city . ', ' . $country;

  $url = 'http://nominatim.openstreetmap.org/search?q=' . urlencode($address) . '&format=json&polygon=0&addressdetails=0';

  $curl = curl_init();
  $timeout = 5;
  curl_setopt($curl, CURLOPT_URL, $url);
  curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
  curl_setopt($curl, CURLOPT_HEADER, 0);
  curl_setopt($curl, CURLOPT_CONNECTTIMEOUT, $timeout);
  $response = curl_exec($curl);
  curl_close($curl);

  $response = json_decode($response);

  $coordinates['lat'] = $response[0]->lat;
  $coordinates['lon'] = $response[0]->lon;
  return $coordinates;
}