PHP if you:
- You like writing PHP.
- Want greater compatibility with web hosts, since 80% of servers have PHP installed.
- Want greater community support a decade ahead of Node (easier to find tutorials and user-friendly PHP packages than JS -- Wordpress > Ghost/Keystone)
Node if you:
- You like writing Javascript.
- Need faster asynchronous action. PHP requires libs like ReactPHP or HHVM.
- Need better CPU performance (although something like Go would be best)
They both can basically do the same thing, just one better than the other in some cases (like async). Just pick which one fits into your tech stack or programming preferences.
If it was all about the best choice for a present problem, the solution would become more clear as you define your scaling issues (ie: "My PHP-based chat app is slow, maybe I should switch to Node and SocketIO").