$client = new GuzzleHttp\Client;

$response = $client->get('https://api.example.com/api/AvailabilitySearch', [
    'headers' => [
        'Authorization' => 'Bearer YOUR_TOKEN_HERE',
    ],
    'form_params' => [
        'VisitDate' => '2017-05-08',
        'PartySize' => '2',
        'ChannelCode' => 'ONLINE',
    ],
]);

// You need to parse the response body
// This will parse it into an array
$response = json_decode($response->getBody(), true);

/////////////////////////////////////////////////////

$endpoint = "http://my.domain.com/test.php";
$client = new \GuzzleHttp\Client();
$id = 5;
$value = "ABC";

$response = $client->request('GET', $endpoint, ['query' => [
    'key1' => $id, 
    'key2' => $value,
]]);

// url will be: http://my.domain.com/test.php?key1=5&key2=ABC;

$statusCode = $response->getStatusCode();
$content = $response->getBody();

// or when your server returns json
// $content = json_decode($response->getBody(), true);