Question

I often hear people say to use “$_SERVER['SERVER_ADDR']“, but that returns the LAN IP of my server (e.g. 192.168.1.100). I want the external IP.

Answer 1

Just query a host that returns your IP address:

$externalContent = file_get_contents('http://checkip.dyndns.com/');
preg_match('/Current IP Address: \[?([:.0-9a-fA-F]+)\]?/', $externalContent, $m);
$externalIp = $m[1];

or, set up a service that simply echoes just the IP, and use it like this:

$externalIp = file_get_contents('http://yourdomain.example/ip/');

Set up the service yourself by simply echoing the remote IP address, or pay someone to host it. Do not use somebody else’s server without permission. Previously, this answer linked to a service of mine that’s now being hit multiple times a second.

Note that in an IP network with one or more NATs, you may have multiple external IP addresses. This will give you just one of them.

Also, this solution of course depends on the remote host being available. However, since there is no widely implemented standard (no ISP and only some home routers implement UPnP), there is no other way to get your external IP address. Even if you could talk to your local NAT, you couldn’t be sure that there isn’t another NAT behind it.