Using Oauth with GuzzleHttp and Magento API in PHP - php

Question

I'm trying to access Magento's REST API in PHP, using Oauth and GuzzleHttp.
I'm basing my example off of this Twitter example:
https://github.com/guzzle/oauth-subscriber
My code so far is:
use GuzzleHttp\Subscriber\Oauth\Oauth1;
use GuzzleHttp\Client;
$client = new Client(array(
'base_url' => 'http://magento',
'defaults' => array('auth' => 'oauth')
));
$oauth = new Oauth1(array(
'consumer_key' => 'magento_key',
'consumer_secret' => 'magento_secret'
));
$client->getEmitter()->attach($oauth);
$response = $client->post('/oauth/initiate');
print_r($response);die();
My host is http://magento. I'm trying to POST my key and secret to /oauth/initiate like is says in the Magento docs.
http://www.magentocommerce.com/api/rest/authentication/oauth_authentication.html#OAuthAuthentication-AuthenticationEndpoints
Does anyone have any insight on what I might be doing wrong? I'm getting a 500 error using Firefox's RESTClient to make this call.
EDIT:
I'm now getting this exception thrown
{
"response": "Client error response [url] http://magento/oauth/initiate [status code] 400 [reason phrase] Bad Request"
}
It's getting a 400 from http://magento/oauth/initiate but this is only when using GuzzleHttp and Oauth. If I make the POST directly within RESTClient as http://magento/oauth/initiate, I get an error response from Magento itself (so it does not 400), and if I add all the necessary params I get the appropriate token responses.
UPDATE:
After inspecting the exception being thrown I'm seeing my authorization array does not conain my callback url. This is suppose to be http://magento/oauth/authorize which I've adde in the RESTClient and it works with the token response. Iv'e tried adding this in my $oauth array as 'callback' and 'oauth_callback' but neither seem to work. I can see this appears in the exception, under my data array, but not in the [authorize] array which I believe it needs to be. I tihnk that's what's missing, if someone has an idea of where it needs to be placed.