PublicKeyCredentialSource.php references "Base64Url\Base64Url", which is not a dependency and not included in the module.
I'm guessing it's supposed to be https://github.com/Spomky-Labs/base64url.
Closed: works as designed
1.0
Code
Nevermind. The issue here is that I didn't have webuthn-lib v3 installed, which bundles base64url.