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

For Guzzle 5, 6 and 7 you do it like this:

use GuzzleHttp\Client;

$client = new Client();

$response = $client->post('url', [
    GuzzleHttp\RequestOptions::JSON => ['foo' => 'bar'] // or 'json' => [...]
]);

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