Can you try removing the following code and try again?
$params['params']['client_id'] = $this->clientID;
$params['params']['clientSecret'] = $this->clientSecret;
The ClientID and ClientSecret should not be passed into the POST https://api.zoom.us/v2/users
endpoint.
Also can you share your use case? If your app is only being used within your Zoom account, using a JWT Token is an easier flow.
Thanks,
Tommy