CloudFlare DDNS updater, inspired by: http://bit.ly/2mqyiwI -- I put this on a server that I know is going to be up 24/7, and pointed my DD-WRT router to it for Dynamic DNS updates.

index.php

<?php
/*
http://SOME_RELIABLE_SERVER/update_cloudflare/CLOUDFLARE_EMAIL/CLOUDFLARE_API-KEY/HOST
...where "update_cloudflare" is a directory containing this file (index.php)

...and the URL slug/segments are parsed with the following regex:
RewriteRule ^/update_cloudflare/([^/]+)?/?([^/]+)?/?(.*) /update_cloudflare/index.php?e=$1&k=$2&h=$3 [L,QSA]

which sends the following GET params:

k: e
v: CLOUDFLARE_EMAIL

k: k
v: CLOUDFLARE_API-KEY

k: h
v: HOST

Note: $dns_id below can be retrieved by changing the action to 'rec_load_all'
	-- you only need to do this one time (for performance).
But for multiple users or hosts, you might like to use a function and store
some data for a quick lookup... or simply do 'rec_load_all' every time.
*/


if(empty($_GET['e']) || empty($_GET['k']) || empty($_GET['h'])) {
	die('missing params');
}


$ip = $_SERVER['REMOTE_ADDR'];
$email = $_GET['e'];
$api = $_GET['k'];
$hostname = $_GET['h'];


$oldIP = gethostbyname($hostname);

// check current IP against host IP
if ($oldIP==$ip) {
	die("IP still set to $ip");
}


// for new users... I will need to retrieve the DNS Record ID, and store it in the DB against the host name.
$dns_id = '12345678'; // insert YOUR domain's ID here
//$action = 'rec_load_all'; // use this to query the full DNS records [ http://www.cloudflare.com/docs/client-api.html#s3.3 ]
$action = 'rec_edit';
$curl_url = 'https://www.cloudflare.com/api_json.html';


$fields = [
	'a' => urlencode($action),
	'z' => urlencode($hostname),
	'email' => urlencode($email),
	'tkn' => urlencode($api)
];


if ($action == 'rec_edit') {
	$fields['type'] = 'A';
	$fields['id'] = urlencode($dns_id);
	$fields['name'] = urlencode($hostname);
	$fields['content'] = urlencode($ip);
	$fields['ttl'] = '1';
	$fields['service_mode'] = '0';
}


//url-ify the data for the POST
$i=0;
$fields_str = '';
foreach($fields as $key=>$value) {
	$fields_str .= $key.'='.$value.((++$i < count($fields)) ? '&' : '');
}


//set the url, number of POST vars, POST data
$csesh = curl_init();
curl_setopt($csesh, CURLOPT_URL, $curl_url);
curl_setopt($csesh, CURLOPT_POST, count($fields));
curl_setopt($csesh, CURLOPT_POSTFIELDS, $fields_str);
curl_setopt($csesh, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($csesh, CURLOPT_HEADER, 0);
curl_setopt($csesh, CURLOPT_RETURNTRANSFER, 1);
$retJSON = curl_exec($csesh);
curl_close($csesh);


$retArr = json_decode($retJSON, true);

if ($retArr["result"]=='success') {
	echo "IP updated to $ip<br/> \n";
} else {
	echo "Houston, we had a problem:<br/> \n";
	print_r($retArr);
}

The previous version put all the data in the CURLOPT_URL, as well as left out CURLOPT_SSL_VERIFYPEER. My host seemed to update cURL which meant I needed to turn off SSL verification. So I also put the POST data into an array for making future changes easier.