未定义的索引：user_image-我缺少什么？

Question

尝试创建照片上传器，但是我不断收到“ user_image”的这些错误，我缺少什么？任何帮助将不胜感激，谢谢：）

错误是：未定义的索引：user_image

PHP代码：

if ($_SERVER['REQUEST_METHOD'] == 'POST') {
        $image_path = mysqli_real_escape_string($conn, 'img/'.$_FILES['user_image']['name']);


        //Make sure file is an image
        if (preg_match("!image!", $_FILES['user_image']['type'])) {
            //copy image to images folder
            if (copy($_FILES['user_image']['tmp_name'], $image_path)) {
                $_SESSION['user_image'] = $image_path;

                $sql = "INSERT INTO users (user_image)";
                    "VALUES('$image_path')";

                //If the query is successful
                if ($sql == true) {
                    $_SESSION['message'] = "Profile picture uploaded.";
                } else {
                    //Error handling
                    $_SESSION['message'] = "Image could not be uploaded.";
                }
            } else {
                $_SESSION['message'] = "File upload failed.";
            }
        }
    } else {
        $_SESSION['message'] = "Please only uplaod JPEG, PNG or GIF files.";
    }

HTML代码：

<form class="form" method="post" enctype="multipart/form-data">
        <div class="container">
         <span class="user"><img src='<?= $_SESSION['user_image']?>'</span>
         </div>
        <div class="user_image"><input type="file" name="user_image" accept="img/*" /></div>
        <button type="submit" name="submit">UPLOAD</button>
      </div>
        </form>