PHP cURL not getting a response...

Hi, I'm really having some trouble getting started with this api... I've written the following to get an auth token but I'm getting nothing back in response from the server at all, not even an error am I missing something?

<?php
      ini_set('display_errors', 'On');      error_reporting(E_ALL);        $headers = array();      $headers[] = 'Authorization: Basic TOKEN';      $headers[] = 'Content-Type: application/x-www-form-urlencoded;charset=UTF-8';           $service_url = 'https://platform.ringcentral.com/restapi/oauth/token';         $curl = curl_init($service_url);         $curl_post_data = array(                    'grant_type'  => 'password',             'username'    => 'username',          'extension'   => 'ext',       'password'    => 'password',              );         curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);         curl_setopt($curl, CURLOPT_POST, true);         curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);      curl_setopt($curl, CURLOPT_POSTFIELDS, $curl_post_data);      $curl_response = curl_exec($curl);         curl_close($curl);           $resp = $curl_response;            print_r($resp);     ?>

Thanks

James