Create a POST request for the client
string|array $uri Resource URI:
array|Collection $headers HTTP headers:
array|Collection|string|EntityBodyInterface $postBody POST body. Can be a string, EntityBody, or: associative array of POST fields to send in the body of the request. Prefix a value in the array with the @ symbol to reference a file.
EntityEnclosingRequestInterface
Guzzle\Http\ClientInterface::createRequest()
public function post($uri = null, $headers = null, $postBody = null);