<?php
$file = 'devices.json';
$data = json_decode(file_get_contents($file), true);

$id = $_POST['id'];

foreach ($data['devices'] as &$device) {
    if ($device['id'] === $id) {
        $device['last_seen'] = time();
        $device['status'] = 'online';
    }
}

file_put_contents($file, json_encode($data, JSON_PRETTY_PRINT));
echo "OK";
?>