How to know which version of the Internet Protocol (IP) a client is using when connecting to my server?

Question

I want to check via php if someone connects to my site via IPv4 or IPv6.

The client address can be found in $_SERVER[\"REMOTE_ADDR\"] but

Answer 1

IPv4 addresses all match the regex /^\d{1,3}(\.\d{1,3}){3,3}$/.