client.php

<?php

class RequestBuilder
{
    /**
     * @var HttpStack
     */
    private $httpStack;

    /**
     * @param HttpStack $httpStack
     */
    public function __construct(HttpStack $httpStack)
    {
        $this->httpStack = $httpStack;
    }

    public function createRequest($method, $uri, $payload): RequestInterface
    {
        $request = $this->httpStack->createRequest($method, $uri);

        if ($payload) {
            $request = $request->withBody($this->httpStack->createStream(json_encode($payload)));
        }

        return $request;
    }
}