There's a million questions on SO about the best way to store IPv4 and IPv6 addresses on a PHP/MySQL setup, but the answers are either out of date (only IPv4) or they're contradictory/incomplete.
I've created this question to go with a clear answer that gives you everything you need to know.
When you only had to worry about IPv4, you could use PHP's ip2long and long2ip to quickly and easily convert an IP address to a integer. These days, however, nearly everyone should expect to come across some IPv6 addresses at some point, and that's something that's only going to increase.
A common and efficient solution to have both is to use PHP's inet_pton function (which handles both IPv4 and v6), and then store the result in a VARBINARY(16) column in your database.
So, let's say we have an IPv6 address like: FE80:0000:0000:0000:0202:B3FF:FE1E:8329
When converted with inet_pton and stored in a VARBINARY(16) column it looks like this:
0xfe800000000000000202b3fffe1e8329
You can then retrieve this from your database and display it using inet_ntop.
Eg:
echo inet_ntop($ipAddressFromDB);
This is a quick and efficient way to store IP addresses in an MySQL database.