[php] How can I use Guzzle to send a POST request in JSON?

You can either using hardcoded json attribute as key, or you can conveniently using GuzzleHttp\RequestOptions::JSON constant.

Here is the example of using hardcoded json string.

use GuzzleHttp\Client;

$client = new Client();

$response = $client->post('url', [
    'json' => ['foo' => 'bar']
]);

See Docs.