updater.php

<?php

// service -> dyndnscustom 
// host -> 

$username = "";
$password = "";
$host = "";

if (empty($_SERVER["PHP_AUTH_USER"]) || $_SERVER["PHP_AUTH_USER"] != $username) {
    die("badauth");
}

if (empty($_SERVER["PHP_AUTH_PW"]) || $_SERVER["PHP_AUTH_PW"] != $password) {
    die("badauth");
}

$public_ip =  $_SERVER['REMOTE_ADDR'];
$current_ip = file_get_contents("ip.txt");

if ($current_ip == $public_ip) {
    die("nochg");
}

$url = "https://api.cloudflare.com/client/v4/zones/..../dns_records/....";
$headers = [
    "Content-Type: application/json",
    "Authorization: Bearer ....",
];

$data = [
    "comment" => "",
    "content" => $public_ip,
    "name" => $host,
    "proxied" => false,
    "ttl" => 60,
    "type" => "A"
];

$ch = curl_init($url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "PUT");
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));

$response = curl_exec($ch);
curl_close($ch);

file_put_contents("ip.txt", $public_ip);
die("ok " . $public_ip);