Invited by Matthew Prince (https://twitter.com/eastdakota/status/453542354117754882), I am switching to Cloudflare DDNS. However it's difficult to configure my dd-wrt router to update my Dynamic DNS host name with Cloudflare. So I ended up with my router calling a PHP script hosted on my website.

cloudflare_ddns_updater.php

<?php
	// ----- dd-wrt DDNS settings: -----
	// DDNS Service: Custom
	// DYNDNS Server: <your web server domain> (e.g. www.example.com)
	// User Name: <anything>
	// Password: <anything>
	// Host Name: <your DDNS hostname> (e.g. home.example.com)
	// URL: /cloudflare_ddns_updater.php?email=<your cloudflare account email>&apikey=<your cloudflare client api key>&domain=<your domain>&host=<your DDNS hostname>
	// (e.g.: /cloudflare_ddns_updater.php?email=me@example.com&apikey=123456789012345678901234567890&domain=example.com&host=home.example.com)
	
	// Input parameters
	$email = empty($_REQUEST['email']) ? '' : $_REQUEST['email'];
	$apikey = empty($_REQUEST['apikey']) ? '' : $_REQUEST['apikey'];
	$domain = empty($_REQUEST['domain']) ? '' : $_REQUEST['domain'];
	$host = empty($_REQUEST['host']) ? $domain : $_REQUEST['host'];
	$newip = empty($_REQUEST['ip']) ? i2c_realip() : $_REQUEST['ip'];
	
	header('Content-Type: text/plain;charset=utf-8');
	
	if (empty($email) || empty($apikey) || empty($domain) || empty($host))
	{
		die('Missing parameter(s), you must specify email, apikey, domain and host');
	}
	
	// Does host matches domain?
	if (strrpos($host, $domain) !== (strlen($host) - strlen($domain)))
	{
		die('Host name do not seems to match domain');
	}
	
	// Do we really need to update?
	$dns = dns_get_record($host, DNS_A);
	if (!empty($dns))
	{
		if (isset($dns[0]['ip']))
		{
			$oldip = $dns[0]['ip'];
			
			if ($oldip == $newip)
			{
				die($host . ' is pointing to ' . $oldip . ' already, no updated needed.');
			}
		}
	}
	
	// Step 1: Get DNS records list from CloudFlare
	$result = cloudflare_api(array(
		'email' => $email,
		'tkn' => $apikey,
		'a' => 'rec_load_all',
		'z' => $domain
	));
	if (!is_array($result) || empty($result['result']) || $result['result'] !== 'success')
	{
		die('Calling rec_load_all for zone "' . $domain . '" failed! CloudFlare response: ' . print_r($result, true));
	}
	$records = $result['response']['recs']['objs'];
	
	// Step 2: Find record for hostname
	$record = null;
	foreach ($records as $rec)
	{
		if (($rec['type'] == 'A') && ($rec['name'] == $host))
		{
			$record = $rec;
			break;
		}
	}
	if (empty($record))
	{
		die('Cannot find DNS record for "' . $host . '". CloudFlare response: ' . print_r($result, true));
	}
	
	// Do we really need to update?
	if ($record['content'] == $newip)
	{
		die($host . ' is set to ' . $record['content'] . ' already, no updated needed. Please be patient while DNS propagates.');
	}
	
	// Step 3: Update host IP address on CloudFlare
	$result = cloudflare_api(array(
		'email' => $email,
		'tkn' => $apikey,
		'a' => 'rec_edit',
		'z' => $domain,
		'type' => $record['type'],
		'id' => $record['rec_id'],
		'name' => $record['name'],
		'content' => $newip,
		'ttl' => $record['ttl'],
		'service_mode' => $record['service_mode']
	));
	if (!is_array($result) || empty($result['result']) || $result['result'] !== 'success')
	{
		die('Calling rec_edit for "' . $host . '" failed! CloudFlare response: ' . print_r($result, true));
	}
	
	echo 'Done';
	exit;

// ---------- Utility functions below ---------- //

function cloudflare_api($data)
{
	// Initialize curl with URL
	$ch = curl_init();
	curl_setopt($ch, CURLOPT_URL, 'https://www.cloudflare.com/api_json.html');
	//curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
	
	// Assign POST data to curl
	curl_setopt($ch, CURLOPT_POST, true);
	curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($data));
	
	// Get response without header
	curl_setopt($ch, CURLOPT_HEADER, 0);
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
	$response = curl_exec($ch);
	
	// Close curl
	curl_close($ch);
	
	$arr = json_decode($response, true);
	
	if (is_null($arr))
	{
		// JSON decode fail, return raw response string
		return $response;
	}
	else
	{
		return $arr;
	}
}

// From http://hk.php.net/source.php?url=/include/ip-to-country.inc
function i2c_realip()
{
	// No IP found (will be overwritten by for
	// if any IP is found behind a firewall)
	$ip = FALSE;
   
	// If HTTP_CLIENT_IP is set, then give it priority
	if (!empty($_SERVER["HTTP_CLIENT_IP"])) {
		$ip = $_SERVER["HTTP_CLIENT_IP"];
	}
   
	// User is behind a proxy and check that we discard RFC1918 IP addresses
	// if they are behind a proxy then only figure out which IP belongs to the
	// user.  Might not need any more hackin if there is a squid reverse proxy
	// infront of apache.
	if (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {

		// Put the IP's into an array which we shall work with shortly.
		$ips = explode (", ", $_SERVER['HTTP_X_FORWARDED_FOR']);
		if ($ip) { array_unshift($ips, $ip); $ip = FALSE; }

		for ($i = 0; $i < count($ips); $i++) {
			// Skip RFC 1918 IP's 10.0.0.0/8, 172.16.0.0/12 and
			// 192.168.0.0/16
			if (!preg_match('/^(?:10|172\.(?:1[6-9]|2\d|3[01])|192\.168)\./', $ips[$i])) {
				if (version_compare(phpversion(), "5.0.0", ">=")) {
					if (ip2long($ips[$i]) != false) {
						$ip = $ips[$i];
						break;
					}
				} else {
					if (ip2long($ips[$i]) != -1) {
						$ip = $ips[$i];
						break;
					}
				}
			}
		}
	}

	// Return with the found IP or the remote address
	return ($ip ? $ip : $_SERVER['REMOTE_ADDR']);
}

?>

nice work @howanghk ! I must admit I hacked mine together rather quickly, so I like that you've gone to the trouble of documenting the DD-WRT settings and other improvements.