I'm going to go against the grain here and say ditch php and build it on node. There is no better way to update your skillset than by building your own product with a tool you've never used. If you're building a web app, you'll need to learn JavaScript regardless.

Worry about scaling problems when you hit them, not when you're picking a language. For comparison, I serve 100k users per day using rails/Postgres on a single $80/month linode server.