1つのファイルで設定画面の練習

index.php

<?php

$user = json_decode(file_get_contents('user.json'), true);

if (isset($_POST['email'])) {
    $email = $_POST['email'];
    $msg = '更新しました。';

    $user = ['email' => $email];
    file_put_contents('user.json', json_encode($user));
} else {
    $email = isset($user['email']) ? $user['email'] : '';
    $msg = '';
}

$email = htmlspecialchars($email, ENT_QUOTES, 'UTF-8');
?>

<div>
<form action="/test/index.php" method="POST">
  <p>
    <label for="email">メールアドレス</label>
    <input type="text" id="email" name="email" value="<?= $user['email'] ?>"/>
  </p>
  <p>
  <?= $msg ?>
  </p>
  <p>
    <input type="submit" value="更新する"/>
  </p>
</form>
</div>

user.json

{"email":"[email protected]"}