Is there a reason to do this?

Wondered what you thought about this. For example, I extended the OpenCart API on a few installations to include various additional functions based on our needs like 'order/get' (which fetches details of an order).

Might be nice to have a custom function for non-standard API endpoints. For example:

public function custom($url = null, $postData = array()) {
    if (empty($url)) throw new InvalidDataException("URL cannot be empty for Order->custom()");
    if (!is_array($postData) throw new InvalidDataException("postData must be an associative array for Order->custom()");
    if (empty($postData['order_id']) throw new InvalidDataException("postData['order_id'] is required for for Order->custom()");

    $this->curl->setUrl($this->oc->getUrl($url.'&order_id='.$postData['order_id']));
    $this->curl->setData($postData);
    $this->curl->makeRequest();

    return $this->curl->getResponse();
}

You may find this a bit of a rare use-case but it could be useful if OpenCart adds additional endpoints that people need and these bindings aren't updated immediately. Gives us a fallback.

I'd be happy to submit a pull request for this if you're interested.

Is there any documentation or demo how to use this or call from another site using curl ?

Changes regarding OpenCart 2.1.0.2

I will list changes as I pass by them in my development if needed. Might take a while.

->login()

• The fieldname for password changed to key.
• The response cookie changed to token.