mysqli_real_escape_string错误

Question

我不知道我做错了什么，但我已经尝试了一切。这是我的代码，它说

  

mysqli_real_escape_string（）：无法在第207行获取mysqli

是

$message = mysqli_real_escape_string($con, strip_tags($_REQUEST['message']));

这是我表单的代码

<form method="POST">
                <input type="text" name="message" placeholder="Message" autocomplete="off"/>
                <input type="submit" name="submit"/>
            </form>
            <?php
                if (isset($_POST['submit']))
                {
                    $message = mysqli_real_escape_string($con, strip_tags($_REQUEST['message']));
                    if($message == null){
                        $warning = "<div class='alert alert-danger'><strong>Error!</strong> You can't leave the message field blank! Fill something in.</div>";
                    }else{
                        $sql = "INSERT INTO messages(Message, Username, User_Rank)VALUES('$message', '$my_username', '$my_rank')";
                        mysqli_query($con, $sql);
                        //UPDATE products SET product_name='52 inch TV', product_code=
                        $sql2 = "UPDATE users SET Total_Messages=Total_Messages + 1 WHERE UID=$my_uid";
                        mysqli_query($con, $sql2);

                        unset($_POST);
                        //header('Location: index.php');
                    }
                }
            ?>