Add support for topics

Thanks for this issue! Actually, this module is only about two days old, so I am still in the process of getting it to work.

The API will change. Actually, I am wondering if I should even include an API in FirebasePhpMessagingService; it might be better for developers to just extend that service to write their own implementations for their projects. And then this module could include an optional service API in a submodule. Any thoughts?

Thank you very much. So I am curious to see how the development of the module progresses.
To your question: Personally, I would like the module to provide a service API for the most frequently required things. Whether this should be outsourced to a submodule is up to you.

ps: I had to delete this code from my version in #2, otherwise it was not working.

else {
  throw new \InvalidArgumentException('Either device token or topic must be provided.');
}

I've seen that you've made a lot of adjustments, just in case that still needs to be taken into account somewhere, possibly already obsolete anyway.

Thank you for the note. If you have a problem with the latest dev version of the module, please open a new issue.

Automatically closed - issue fixed for 2 weeks with no activity.