Adding User to Chef via Chef Server API - php

Question

I am attempting to create a user on my Chef Server through the Chef Server API. I am currently using a PHP wrapper for the API interface which can be found here.
I understand that in order to create a user using the API, you must provide the pivotal.pem file as well as the pivotal username.
Here is a snippet of my PHP code that I am using:
// ENVIRONMENT VARIABLES
// The URL for the Chef Server
$server = "mycompany.com";
// The name used when authenticating
$client = 'pivotal';
// The location of the file which contains the client key
$key = "../../../pivotal.pem";
// The version of the Chef Server API that is being used
$version = "12.0.2";
// Create a Chef object
$chef = new Chef($server, $client, $key, $version);
// API Request
$createUserRequest = array("name"=>$displayName, "display_name"=>$displayName, "email"=>$emailAddress, "password"=>$password);
$createUserRequest = json_encode($createUserRequest);
$chef->post('/users/', $createUserRequest);
Here is the reponse that I receive from the server:
Fatal error: Uncaught exception 'Exception' with message 'Invalid signature for user or client 'pivotal'' in C:\Workspace\SA_SSAF\app\plugin\CHEF\Chef PHP\src\Jenssegers\Chef\Chef.php:187
Stack trace:
#0 C:\..\..\app\plugin\CHEF\Chef PHP\src\Jenssegers\Chef\Chef.php(70): Jenssegers\Chef\Chef->api('/users/', 'POST', '{"name":"entra"...')
#1 C:\..\..\app\plugin\CHEF\controller\chefEnrollUser.php(55): Jenssegers\Chef\Chef->post('/users/', '{"name":"entra"...')
#2 {main}
thrown in C:\..\..\app\plugin\CHEF\Chef PHP\src\Jenssegers\Chef\Chef.php on line 187
If I try and perform a GET request using the same configuration, I receive a successful response.
Any clues as to why something like this may be happening?
Thanks