public static function getAccessToken($code, $redirect_uri)
{
$params = array('client_id' => self::$client_id, 'client_secret' => self::$client_secret, 'grant_type' => 'authorization_code', 'code' => $code, 'redirect_uri' => $redirect_uri);
return self::$accessTokenURL . '?' . http_build_query($params);
}