Thank you very much for the reply. I've been playing around with the API all day today and I think you're absolutely spot on with regards to it being the best way to go about doing what I want to achieve. I can stick to my PHP backend and call the relevant functions as I need to.
I'm still not 100% sure if manually creating/updating/deleting user accounts is the way forward, and I don't think I can do so with web hooks or slash commands, but we shall see.
Once again thank you kindly for the reply, it's certainly helped me realise what needs to be done.
Mike