Not understanding mocking in php or in guzzel.

Looking for some help here. Consider the following class:

class Details {

    public function details($href) {
        $client = new Client();

        $response = $client->request('GET', $href);
        EveLogHandler::requestLog($response, 'eveonline_item_details.log');

        if ($response->getStatusCode() === 200) {
            return json_decode($response->getBody()->getContents());
        }

        return false;
    }
}

Pretty basic, we create a client, send a GET request to the href, log the response, then do stuff on status code of 200 or return false.

What I dont get is the following:

1. How do you test this with out hitting the API?
2. How do you mock the client objects as demonstrated here inside a class method
3. What if this was a pool? How would you mock a pool? see example bellow:

In this example for Pool I have a method inside of a different class that uses the pool concept:

public function fetchGroupsInfromation(array $groups) {
    $groupInformationContainer = [];

    $createdRequests           = [];
    $acceptedResponses         = [];
    $rejectedResponses         = [];

    $client                    = new Client();

    foreach($groups as $group) {
        array_push($createdRequests, new Request('GET', $group->types->href));
    }

    $pool = new Pool($client, $createdRequests, [
        'concurrency' => 10,
        'fulfilled'   => function ($response, $index) use (&$acceptedResponses) {
            EveLogHandler::requestLog($response, 'eve_online_group_items_responses.log');

            $groupPagesItterator = new GroupPagesItterator(json_decode($response->getBody()->getContents()));
            $acceptedResponses[$index] = iterator_to_array($groupPagesItterator->getAllPages());
        },
        'rejected'    => function ($reason, $index) use(&$rejectedResponses)  {
            array_push($rejectedResponses, $reason);
        },
    ]);

    $promise = $pool->promise();
    $promise->wait();
}

I took out some of the other logic inside this method because the main thing I need help with is understanding how to write tests for class methods like these where they use third party services.

The goal is to never hit the API, I have a couple controller tests further up that do that for me (integration tests if you will) How ever I need to test code like this to make sure I get back what I expect. But I feel that if I mock the method or the code inside the method that at that point I am just "spell checking" and I really need to make sure that ok when I pass you this, I ned x back or I need y back.

So can you guys help me buy maybe giving me some examples, maybe some links to examples on how this type of code would be tested? Some real world stuff?