If you don't specify the http_version at all, the curl client will let curl decide which version it uses.

That doesn't seem to be the case from my findings. Or at least, it won't prefer HTTP/3 over HTTP/2.

On cli:

$ curl -I https://www.denotenshop.nl/
HTTP/2 200

$ curl --http3 -I https://www.denotenshop.nl/
HTTP/3 200

So it's not the default one even if the website supports it.

Same thing in PHP:

use Symfony\Component\HttpClient\CurlHttpClient;

$httpClient = new CurlHttpClient();
$httpResponse = $httpClient->request('GET', 'https://www.denotenshop.nl/');
$httpResponse->getHeaders();
print_r($httpResponse->getInfo());

Results in following output:

Array
(
    [response_headers] => Array
        (
            [0] => HTTP/2 200
            [1] => server: nginx
...

Changing versions in the options works, but not for version '3.0' because there is no support yet in the http client:

$httpResponse = $httpClient->request('GET', 'https://www.denotenshop.nl/', ['http_version' => '1.1']);
// => output: HTTP/1.1 200 OK
$httpResponse = $httpClient->request('GET', 'https://www.denotenshop.nl/', ['http_version' => '2.0']);
// => output: HTTP/2 200
$httpResponse = $httpClient->request('GET', 'https://www.denotenshop.nl/', ['http_version' => '3.0']);
// => output: HTTP/2 200

And yes, my curl in php is seen as being compatible with HTTP/3:

$ php -i | grep -A30 curl
--
curl

cURL Information => 8.9.1
...
HTTP3 => Yes
...
--

Or maybe we should wait with implementing it until curl no longer considers the feature to be experimental, maybe they will change the default/fallback logic later on when they consider the implementation stable...

Update: found this thread, so probably we'll have to wait until PHP 8.4 is out.