pfSense

The default service checkip.dyndns.org will fail when connecting over IPv6 (e.g. if upstream only provides IPv6 connectivity). The default service could be changed to e.g. noip.com (see https://www.noip.com/integrate/ip-detection) and updated to prefer IPv4 and fail back to IPv6.

Test:

# PHP code
$ip_address = get_interface_ip('wan');
$ip_ch = curl_init('http://ip1.dynupdate6.no-ip.com/');
curl_setopt($ip_ch, CURLOPT_VERBOSE, 1);
$curlerr = fopen('php://temp', 'w+');
curl_setopt($ip_ch, CURLOPT_STDERR, $curlerr);
curl_setopt($ip_ch, CURLOPT_RETURNTRANSFER, 1);
$ip_result_page = curl_exec($ip_ch);
curl_close($ip_ch);
$ip_result_decoded = urldecode($ip_result_page);
rewind($curlerr);
$curlerrlog = stream_get_contents($curlerr);
fclose($curlerr);
print_r($ip_result_decoded ? $ip_result_decoded : $curlerrlog);

# success
2001:db8:x:x:x:x:x:x

# fail (using http://checkip.dyndns.org/ as the URL)
*   Trying 158.101.44.242:80...
* Immediate connect fail for 158.101.44.242: Network is unreachable
*   Trying 193.122.6.168:80...
* Immediate connect fail for 193.122.6.168: Network is unreachable
*   Trying 132.226.8.169:80...
* Immediate connect fail for 132.226.8.169: Network is unreachable
*   Trying 132.226.247.73:80...
* Immediate connect fail for 132.226.247.73: Network is unreachable
*   Trying 193.122.130.0:80...
* Immediate connect fail for 193.122.130.0: Network is unreachable
* Closing connection 0