firebase-php: Fatal error during initialization of auth object

Describe the bug Hi, I ran into a error during initialization of the auth object. This error comes up:

Fatal error: Declaration of Kreait\Firebase\Auth\ApiClient::request($method, $uri, array $options = Array): Psr\Http\Message\ResponseInterface must be compatible with GuzzleHttp\ClientInterface::request(string $method, $uri, array $options = Array): Psr\Http\Message\ResponseInterface in {path}/libs/vendor/kreait/firebase-php/src/Firebase/Auth/ApiClient.php on line 25

To Reproduce I followed the tutorial in the documentation here step by step.

I downloaded the composer. Then I used it to download this set of libraries. I moved the created directories through ftp onto the server. There I tried to run this code and I got the error.

require "../libs/vendor/autoload.php";
use Kreait\Firebase\Factory;
use Kreait\Firebase\Auth\Token\Exception\InvalidToken;
use Kreait\Firebase\ServiceAccount;

if(isset($_POST['idTokenString'])) {
     $idTokenString  = $_POST['idTokenString'];
		
     //create Firebase factory object
     $factory = (new Factory())->withServiceAccount('../privatekey.json');
     $auth = $factory->createAuth(); //this line makes the error
}

Expected behavior It should run without an error.

Environment (please complete the following information):

• OS: Windows 10
• PHP version: 7.1.33-15+0~20200419.36+debian8~1.gbp2384b3
• Firebase SDK Version: 5.7

Other info Sorry if this isn’t the place where to place error, but I don’t know where else to post it. Thanks for help in advance.