https://www.cloudflare.com/api.html?a=DIUP&u=<my_login>&tkn=<my_token>&ip=<my_ip>&hosts=<my_record>

Tue Jan 24 00:36:47 2012: INADYN: Started 'INADYN Advanced version 1.96-ADV' - dynamic DNS updater.
Tue Jan 24 00:36:47 2012: I:INADYN: IP address for alias '<my_record>' needs update to '<my_ip>'
Tue Jan 24 00:36:48 2012: W:INADYN: Error validating DYNDNS svr answer. Check usr,pass,hostname! (HTTP/1.1 303 See Other
Server: cloudflare-nginx
Date: Mon, 23 Jan 2012 14:36:48 GMT
Content-Type: text/plain
Connection: close
Expires: Sun, 25 Jan 1981 05:00:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Location: https://www.cloudflare.com/api.html?a=DIUP&u=<my_login>&tkn=<my_token>&ip=<my_ip>&hosts=<my_record>
Vary: Accept-Encoding
Set-Cookie: __cfduid=<id>; expires=Mon, 23-Dec-2019 23:50:00 GMT; path=/; domain=.cloudflare.com
Set-Cookie: __cfduid=<id>; expires=Mon, 23-Dec-2019 23:50:00 GMT; path=/; domain=.www.cloudflare.com

You must include an `a' paramiter, with a value of DIUP|wl|chl|nul|ban|comm_news|devmode|sec_lvl|ipv46|ob|cache_lvl|fpurge_ts|async|pre_purge|minify|stats|direct|zone_check|zone_ips|zone_errors|zone_agg|zone_search|zone_time|zone_grab|app|rec_se

DDNS Service: Custom
DYNDNS Server: <yourserver.com>
Username: <anything>
Password: <anything>
Host Name: <your domain name to update on cloudflare>
URL: /cloudflare_update.php?key=<your secret key>&hostname=

if(empty($_GET['key']) || ($_GET['key'] != "<your secret key>")) die;
$hostname = $_GET['hostname'];
$ip = $_SERVER['REMOTE_ADDR'];
$username = "<yourcloudflareusername@email.com>";
$api = "<your cloudflare api key>";
$curl = "https://www.cloudflare.com/api_json.html?a=DIUP&hosts=$hostname&u=$username&tkn=$api&ip=$ip";
$ch = curl_init($curl);
curl_exec($ch);
curl_close($ch);