src/Core/OAuth/OAuth2/OAuth2AccessToken.php
public function getRefreshToken(){
        if(isset($this->refresh_token) && !empty($this->refresh_token))  return $this->refresh_token;
        else throw new SdkException("The OAuth 2 Refresh Token is not set in the Access Token Object.");
    }
public function getRefreshToken(){
        if(isset($this->refresh_token) && !empty($this->refresh_token))  return $this->refresh_token;
        else throw new SdkException("The OAuth 2 Refresh Token is not set in the Access Token Object.");
    }
Return the refresh Token
@return String