更新用户名和电子邮件

Question

我正在尝试从用户会话中更新用户详细信息，我不断收到“致命错误：未捕获的错误：在bool中调用成员函数bind_param（）的错误”，我认为这是指事实SQL语句未作为字符串读取，因此未插入数据库中。有人可以帮忙吗？新手。谢谢

// PHP

if (isset($_POST['update-btn'])) {
    $username = $_POST['username'];
    $email = $_POST['email'];

    //validation

    if (empty($username)) {
        $errors['username'] = "Username required!";
    }
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        $errors['email'] = "Email is invalid!";
    }

    if (count($errors) === 0) {
        $sql = "UPDATE user SET username=?, email=? WHERE id=?";
        $stmt = $db->prepare($sql);
        $stmt->bind_param('ssi', $username, $email, $_SESSION['id']);

        if ($stmt->execute()) {
            $_SESSION['id'] = $user['id'];
            $_SESSION['username'] = $username;
            $_SESSION['email'] = $email;
            $_SESSION['message'] = "Your account details have been updated";
            exit();
        } else {
            $errors = "Detail could not be updated.";
        }
    }
}

// HTML

     <form action="update.php" method="post"> .          

      <div class="form-group">
        <div class="form-label-group">
        <input type="text" name="username" id="username" value="<? 
           php echo $_SESSION['username'];?>" class="form-control" 
            placeholder="Name" autocomplete="off" 
             autofocus="autofocus" required="true">
        <label for="username">Username</label>
    </div>
    <br>
    <div class="form-label-group">
        <input type="email" name="email" id="email" value="<? 
                        php echo $_SESSION['email'];?>" class="form- 
                          control" placeholder="Email" 
         autocomplete="off" autofocus="autofocus" required="true">

        <label for="email">Email</label>
    </div>
</div>
        <input class="btn btn-primary btn-block" id="update-
                     btn" name="update-btn" type="submit" 
      value="Update">
       </div>
     </div>

     </form>

我希望在数据库中更新用户信息。