A simple IP address reflector.

gistfile1.php

<?php
// Think something like http://whatismyipaddress.com, but machine parsable,
// small, and fast(ish). Returns a json encoded hash.
// Will accept a port & optional timeout parameter. Attempts to make a TCP 
// connection back to the host on specified port. Useful to see if you're 
// externally reachable.
// I'm surprised noone else got here first. - donavanm

// Really PHP? What year is this again?
function get_host($ip){
        $ptr= implode(".",array_reverse(explode(".",$ip))).".in-addr.arpa";
        $host = dns_get_record($ptr,DNS_PTR);
        if ($host == null) return $ip;
        else return $host[0]['target'];
}

function test_port($host, $port, $timeout) {
    $fp = @fsockopen($host, $port, $errno, $errstr, $timeout);
    if (!$fp) {
        return false;
    } else {
        fclose($fp);
        return true;
    }
}

// Get the client info
$host['ipaddress'] = $_SERVER["REMOTE_ADDR"];
if ($_SERVER['REMOTE_HOST']) {
    $host['fqdn'] = $_SERVER['REMOTE_HOST'];
} else {
    $host['fqdn'] = get_host($host['ipaddress']);
}

// Check for port parameter & test connection
if(!empty($_GET["port"]))  {
    if(!empty($_GET["timeout"]) && $_GET["timeout"] <= 3 ) { $timeout = $_GET["timeout"]; }
    else { $timeout = 0.5; }
    $port = $_GET["port"];
    $host['port'][$port] = test_port($host['ipaddress'], $port, $timeout);
}

// setup some headers
header("Expires: Thu, 19 Nov 1981 08:52:00 GMT");
header('Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0');
header("Content-type: application/json");

// Send body
echo json_encode($host);
?>