/// PRECONDITION for the test: two servers with both IPv4 AND IPv6 address ///

/// SCRIPT RETURNING YOUR REMOTE ADDRESS ///
/// PLACE THIS ONE ON SERVER 1 ///
<?php
$ip = $_SERVER['REMOTE_ADDR'];
$ts = isset($_GET['ts']) ? $_GET['ts'] : null;
$v = isset($_GET['v']) ? $_GET['v'] : null;
$str = "IP: " . $ip . " ts: " . $ts . " v: " . $v;
echo $str;
?>

//// TEST-SCRIPT ////
//// PLACE THIS SCRIPT ON SERVER 2 and edit your.host ////
<?php
function get_remote_ip ($v)
{
        $ch = curl_init();
        $url = "your.host/myip.php?ts=" . rand(1000,5000) . "&v=" . $v;
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($ch, CURLOPT_FRESH_CONNECT, 1);

        if($v == 4) {
                curl_setopt($ch, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4 ); }
        elseif($v == 6) {
                curl_setopt($ch, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V6 ); }
        else {
                curl_setopt($ch, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_WHATEVER );  }

        $ret = curl_exec($ch);
        curl_close($ch);
        unset($ch);
return $ret;
}

$whatever = get_remote_ip("1");
$v4 = get_remote_ip("4");
$v6 = get_remote_ip("6");

echo "whatever: $whatever";
echo "<br />";
echo "v4: $v4";
echo "<br />";
echo "v6: $v6";
echo "<br />";
?>