You could scale the API with Kubernetes and MySQL with Vitess off-the-shelf or architect the sharding yourself. My only concern with MySQL would be that, contrary to PostgreSQL, you can’t rollback DDL statements which makes migrations a PITA, but this is true at all stages and unrelated to scaling. I find it interesting PHP was mentioned since it gets a bad wrap. I personally think it isn’t that great of a programming language but if it’s what you’re familiar with then more power to you. It’s like if someone said you could articulate ideas in French much better than in English, because it has words for things that English doesn’t, I’d still write a better book in English than French. Although, if you understand the fundamentals of programming languages, then learning a new one, especially a high-level OOP one, doesn’t take that long.