bind_param()问题

我遇到了bind_param函数的问题.我将在下面发布所有信息.

错误:

Fatal error: Call to a member function bind_param() on a non-object in /home4/lunar/public_html/casino/blogpost.php on line 88

MySQL错误:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ':user, :title, :message, :image, :category, NOW())' at line 1

查询:

    $user = $_COOKIE['user'];
    $title = $_POST['title'];
    $message = $_POST['message'];
    $image = $_POST['image'];
    $category = $_POST['category'];
    $stmt = $mysqli->prepare("INSERT INTO `lunar_casino`.`posts` (`id`, `by`, `title`, `message`, `image`, `category`, `date`) VALUES(NULL, :user, :title, :message, :image, :category, NOW())");
    echo $mysqli->error;
    $stmt->bind_param(":user", $user);
    $stmt->bind_param(":title", $title);
    $stmt->bind_param(":message", $message);
    $stmt->bind_param(":image", $image);
    $stmt->bind_param(":category", $category);
    $stmt->execute();
    if(!$stmt){
      echo "There has been an error with our database! Please contact the website administrator!

";
      echo $mysqli->error;
    } else {
      echo "You have successfully added a blog post!

";
    }

有什么想法为什么会这样？