Still getting the string error on the $json with the full script using swentel\nostr\Relay\Relay
Fixed / new release is out.
https://github.com/nostrver-se/nostr-php/releases/tag/1.3.3
$ bin/nostr-php --content "Hello world! Testing this fix" --key privkey --relay wss://nostr.sebastix.dev
Send to Nostr!
Discussion
Switched to RelaySet and it seems to work, but it hangs after writing and throws a fatal error.
PHP Fatal error: Uncaught WebSocket\Exception\ConnectionClosedException: Connection has unexpectedly closed in /home/.../vendor/phrity/websocket/src/Connection.php:374
I didn't have access to a relay. Works now.