get_visitor_ip.php

<?php

// https://stackoverflow.com/a/2031935/4688612
// https://stackoverflow.com/q/67277544/4688612
protected function get_visitor_ip(): string
{
    $proxy_headers = [
        'HTTP_CF_CONNECTING_IP', // Cloudflare
        'HTTP_TRUE_CLIENT_IP', // Cloudflare Enterprise
        'HTTP_INCAP_CLIENT_IP', // Incapsula
        'HTTP_X_SUCURI_CLIENTIP', // Sucuri
        'HTTP_FASTLY_CLIENT_IP', // Fastly
        'HTTP_X_FORWARDED_FOR', // any proxy
        'HTTP_X_FORWARDED',
        'HTTP_X_CLUSTER_CLIENT_IP',
        'HTTP_FORWARDED_FOR',
        'HTTP_FORWARDED',
        'HTTP_CLIENT_IP',
        'REMOTE_ADDR'
    ];

    foreach ($proxy_headers as $key) {
        if (array_key_exists($key, $_SERVER) === true) {
            foreach (explode(',', $_SERVER[$key]) as $ip) {
                $ip = trim($ip); // just to be safe

                if (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) !== false) {
                    return $ip;
                }
            }
        }
    }

    return '';
}