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

What about counting the number of '.' and/or ':' in $_SERVER["REMOTE_ADDR"] ?

If there is more than 0 ':', and no '.' symbol in $_SERVER["REMOTE_ADDR"], I suppose you can consider you user is connected via IPv6.

Another solution might be to use the filter extension : there are constants (see the end of the page) that seem to be related to IPv4 and IPv6 :

FILTER_FLAG_IPV4 (integer)
Allow only IPv4 address in "validate_ip" filter.

FILTER_FLAG_IPV6 (integer)
Allow only IPv6 address in "validate_ip" filter.