cloudflare.php

<?php
/**
 * DDNS Service: Custom
 * DYNDNS Server: <yourserver.com>
 * Username: 
 * Password: 
 * Host Name: 
 * URL: /cloudflare_update.php?key=&hostname=
 */
/**
 * Settings
 */
define('API_KEY', $_GET['apikey']); // CloudFlare API key
define('USERNAME', 'user@example.com'); // Email address used to login into CloudFlare
define('IP', getCurrentIP()); // Current IP address

echo 'Setting IP address to "' . IP . '"...<br />';

// Your web sites:
update('example.com');
update('example.net');

/**
 * @return string Current IP address
 */
function getCurrentIP()
{
    $source = "http://api.externalip.net/ip/";
    return file_get_contents($source);
}

/**
 * This function sends request to CloudFlare to change IP address of an A record with specific name
 * @param $hostname Name of the record that needs to be updated
 */
function update($hostname)
{
    echo "<br />Updating $hostname...<br />";
    $update_url = 'https://www.cloudflare.com/api_json.html?a=DIUP&hosts='
        . $hostname . '&u=' . USERNAME . '&tkn=' . API_KEY . '&ip=' . IP;
    echo file_get_contents($update_url);
}