<?php
declare(strict_types=1);

$ip = $_SERVER['REMOTE_ADDR'] ?? 'unknown';

echo <<<HTML
<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<title>Your remote IP address</title>
<style>
div {
  text-align: center;
  font-size: large;
}
</style>
</head>
<body>
<p>社内専用</p>
<p>下のIPアドレスをお知らせください</p>
<div>
HTML;

echo htmlspecialchars($ip, ENT_QUOTES, 'UTF-8');

echo <<<HTML
</div>
</body>
</html>
HTML;