Logging up to a server your local IP obtained via DHCP

get_ip.php

<?php
// On the server to get list of logged IPs
$content = file_get_contents("/tmp/ip.html");
echo $content;

post_ip.php

<?php
// ON THE SERVER
$html_file = "/tmp/ip.html";
$data = json_decode( file_get_contents('php://input') );
$error = false;
header('Content-Type: application/json; charset=utf-8');
if(!empty($data)){
    if(isset($data->host) && isset($data->ip)){
        $date = date("F jS, Y", strtotime("now"));
        $ip = $_SERVER["REMOTE_ADDR"];
        $str = "<strong>$data->host</strong>: <span>$data->ip [$ip]</span> <em>$date</em><br />".PHP_EOL;
        $flags = 0;
        if(file_exists($html_file)){
            $flags = FILE_APPEND;
        }
        file_put_contents($html_file, $str, $flags);
        echo json_encode(['success' => true]);
        echo PHP_EOL;
    } else {
        $error = true;
    }
} else {
    $error = true;
}
if($error){
echo json_encode(['success' => false]);
echo PHP_EOL;
}

rc.local

# ON THE CLIENT
# don't forget to do 
# sudo chmod +x /etc/rc.local
...
hostname=$(hostname -f)
ip=$(hostname -I)
curl -X POST https://<your server>/network-tools/post_ip.php -H 'Content-Type: application/json' -d "{\"host\":\"$hostname\",\"ip\":\"$ip\"}"
...