使用PHP问题上传文件

Question

我正在尝试编写一个用户可以上传文件的小脚本，但出于某种原因，我遇到了问题。出于某种原因，当单击提交按钮时，_$FILES数组将返回NULL（使用var_dump()和var_export()进行测试），所以当然其他所有内容都被跳过，我得到“错误：没有上传文件“我无法确定这是为什么。下面是表单和PHP脚本的HTML代码段。

HTML

<form class='admin_form' action='' method='POST' enctype='multipart/form-data'>
    <fieldset>
       <label for='comic_page'>Filename</label>
       <input type='file' name='uploaded_file' id='uploaded_file'> 
       <input type='submit' name='comic_page_btn' value='Upload'>

       <span id='comic_page_upl_err_msg'>".$err_msg."</span>
    </fieldset>
</form>

PHP

if(isset($_POST['comic_page_btn'])){

    echo var_dump($_FILES['uploaded_file']['name']).'<br>';
    echo var_export($_FILES["uploaded_file"]);
    if((!empty($_FILES["uploaded_file"])) && ($_FILES['uploaded_file']['error'] == 0)) {
        //Check if the file is JPEG image and it's size is less than 350Kb
        $filename = basename($_FILES['uploaded_file']['name']);
        $ext = substr($filename, strrpos($filename, '.') + 1);
        if (($ext == "jpg") && ($_FILES["uploaded_file"]["type"] == "image/jpeg") && 
          ($_FILES["uploaded_file"]["size"] < 350000)) {
          //Determine the path to which we want to save this file
            $newname = dirname(__FILE__).'/uploads/'.$filename;
            //Check if the file with the same name is already exists on the server
            if (!file_exists($newname)) {
              //Attempt to move the uploaded file to it's new place
              if ((move_uploaded_file($_FILES['uploaded_file']['tmp_name'],$newname))) {
                $err_msg = "The file has been saved as: ".$newname;
              } else {
                 $err_msg = "Error: A problem occurred during file upload!";
              }
            } else {
               $err_msg = "Error: File ".$_FILES["uploaded_file"]["name"]." already exists";
            }
        } else {
           $err_msg =  "Error: Only .jpg images under 350Kb are accepted for upload";
        }
      } else {
            $err_msg = "Error: No file uploaded";

      }
}