生成一个登录/注册表单
<html>
<head>
  <title>登录/注册表单</title>
</head>
<body>
  <!-- 登录现有用户的表单 -->
  <h1>登录</h1>
  <form action="process_login.php" method="post">
    <div>
      <label for="username">用户名：</label>
      <input type="text" name="username" id="username">
    </div>
    <div>
      <label for="password">密码：</label>
      <input type="password" name="password" id="password">
    </div>
    <input type="submit" name="submit" value="登录">
  </form>
  <!-- 创建新账户的表单 -->
  <h1>注册</h1>
  <form action="process_signup.php" method="post">
    <div>
      <label for="username">创建一个用户名：</label>
      <input type="text" name="username" id="username">
    </div>
    <div>
      <label for="password">创建一个密码：</label>
      <input type="password" name="password" id="password">
    </div>
    <div>
      <label for="email">输入您的电子邮件：</label>
      <input type="text" name="email" id="email">
    </div>
    <input type="submit" name="submit" value="注册">
  </form>
</body>
</html>