我已经盯着这个问题看了好几个小时了。就我的一生而言，我找不到任何解释。PHP找不到userid和subgenre元素。其他的都找到了。

HTML表单

<form action="php/writers.php" method="post" enctype="multipart/form-data" target="upload-target" onsubmit="startUpload();" >
    <h1>Writers</h1>
    <p>
        <input id="title" name="title" class="form-control"  placeholder="Title" required autofocus="true" />
    </p>
    <p>
        <label for="work-type" class="fixed50">Form:</label>
        <select id="work-type" name="work-type">
            <option value="fiction">Fiction</option>
            <option value="non-fiction">Non-Fiction</option>
            <option value="screenplay">Screenplay</option>
            <option value="play">Play</option>
        </select>
    </p>
    <p>
        <p>For an explanation of the genres shown here, see <a target="_blank" href="https://en.wikipedia.org/wiki/List_of_genres">List of genres</a></p>
        <label for="genre"  class="fixed50">Genre:</label>
        <select id="genre" name="genre">
        </select>
    </p>
    <p>
        <label for="subgenre" class="fixed50">Sub-Genre:</label>
        <select id="subgenre" name="subgenre">`
            <option value="0">None</option>
        </select><br/>
    </p>
    <p>
        <label for="nbrPages" class="fixed50">Number of pages:</label>
        <input id="nbrPages" name="nbrPages" required style="width: 48px" placeholder="Pages" /><br/>
    </p>
    <p>For a limied time, writers can upload their sysnopsis or query letterr for <span style="color: #f00; font-weight:bold">FREE</span>. We reserve the right to change this policy without notice.</p>
    <div id="tips">The objective of a sysnopsis or query letter is to entice enablers into requesting your manuscript. 
                    It must be brief and to the point, now more than twp pages and, of course very well written.
    </div>
    <div class="form-group">
        <p id="file-warning">Your synopsis or query letter must be a PDF file.
        <a target="_blank" href="https://www.freepdfconvert.com/" target="_blank">Free file conversion to PDF</a></p>
    </div>
    <div class="form-group">
        <label for="file2upload">PDF to Upload</label>
        <input id="file2upload" name="file2upload" type="file" required value="File to upload" />
    </div>
    <div id="recaptcha-elements"></div>
    <div class="form-group">
        <button type="submit" id="writers-submit" class="btn btn-default">Submit</button>
    </div>
    <input id="userid" name="userid" type="hidden" />
    <iframe id="upload-target" name="upload-target" src="#" ></iframe>                 
</form>

下面的JS函数将正确的值写入控制台。

JS

function startUpload() {
    $("#userid").val(window.localStorage.getItem('user-id'));
    console.log("jquery val=" + $("#subgenre").val());
    console.log("getElementById value=" + document.getElementById('subgenre').value);
    showMessage(1, "Uploading...");
    return true;
}

控制台日志

userid=51 VM39985:122

jquery val=0 VM39985:123

getElementById值=0

/php/writers.php：1 POST 500（内部服务器错误）

PHP

$uploadDirectory = '/home/deje/public_html/writers-tryst/uploads/'; //specify upload directory ends with / (slash)
require_once "dbconnect.php";
$result = 0;
$File_Name          = basename($_FILES['file2upload']['name']);
$File_Ext           = substr($File_Name, strrpos($File_Name, '.')); //get file extention
$Random_Number      = rand(0, 9999999999); //Random number to be added to name.
$NewFileName        = $Random_Number.$File_Ext; //new file name
var_dump($_REQUEST); 
$target_path = $uploadDirectory . $NewFileName;
if (@move_uploaded_file($_FILES['file2upload']['tmp_name'], $target_path)) {
    $finfo = finfo_open(FILEINFO_MIME_TYPE);
    $mime_type = finfo_file($finfo, $uploadDirectory . $NewFileName);
    finfo_close($finfo); 
    if ($mime_type != 'application/pdf') {
        unlink($UploadDirectory . $NewFileName);
        $data = array('File MUST be a PDF!');
        $result = 0;
    } else $result = 1;
}
if (!isset($_REQUEST["title"]) || empty(trim($_REQUEST["title"])))
    throw new Exception('You must enter a title.');       
else {
    $title = filter_var(trim($_REQUEST["title"]), FILTER_SANITIZE_STRING);
    $title = htmlspecialchars_decode($title, ENT_QUOTES);
}
if (!isset($_REQUEST["userid"]) || empty(trim($_REQUEST["userid"])))
    throw new Exception('Userid is missing.');       
else {
    $userrid = filter_var(trim($_REQUEST["userid"]), FILTER_SANITIZE_STRING);
    $userid = htmlspecialchars_decode($userid, ENT_QUOTES);
}
if (!isset($_REQUEST["work-type"]) || empty(trim($_REQUEST["work-type"])))
    throw new Exception('You must enter a work type.');       
else {
    $worktype = filter_var(trim($_REQUEST["work-type"]), FILTER_SANITIZE_STRING);
    $worktype = htmlspecialchars_decode($worktype, ENT_QUOTES);
}
if (!isset($_REQUEST["genre"]) || empty(trim($_REQUEST["genre"])))
    throw new Exception('You must enter a title.');       
else {
    $genre = filter_var(trim($_REQUEST["genre"]), FILTER_SANITIZE_STRING);
    $genre = htmlspecialchars_decode($genre, ENT_QUOTES);
}
if (!isset($_REQUEST["subgenre"]) || empty(trim($_REQUEST["subgenre"])))
    throw new Exception('You must enter a sub-genre.');       
else {
    $subgenre = filter_var(trim($_REQUEST["subgenre"]), FILTER_SANITIZE_STRING);
    $subgenre = htmlspecialchars_decode($subgenre, ENT_QUOTES);
}
if (!isset($_REQUEST["nbrPages"]) || empty(trim($_REQUEST["nbrPages"])))
    throw new Exception('You must enter the number of pages your work contains.');       
else {
    $nbrPages = filter_var(trim($_REQUEST["nbrPages"]), FILTER_SANITIZE_STRING);
    $nbrPages = htmlspecialchars_decode($nbrPages, ENT_QUOTES);
}
$dbh = connect2DB();
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$stmt = $dbh->prepare(
    "INSERT Writers(fkAccounts, Title, WorkType, Genre, SubGenre, Filename) 
    VALUES(:fk, :title, :worktype, :genre, :subgenre, :filename)"
);
$stmt->bindParam(':fk', $userid, PDO::PARAM_INT, 10);
$stmt->bindParam(':title', $title, PDO::PARAM_STR, 255);
$stmt->bindParam(':worktype', $worktype, PDO::PARAM_STR, 30);
$stmt->bindParam(':genre', $genre, PDO::PARAM_STR, 100);
$stmt->bindParam(':subgenre', $subgenre, PDO::PARAM_STR, 100);
$stmt->bindParam(':filename', $NewFileName, PDO::PARAM_STR, 30);
$stmt->execute();

echo "<script type='text/javascript'>stopUpload(" . $result . ");</script>";
sleep(1);
exit();
$data = array();
require_once 'cookies.php';
if (isset($_REQUEST['files'])) {  
    $error = false;
    $files = array();
    foreach ($_FILES as $file) {
        if ($file["size"] > $filesize_limit) {
//            $data = array('Maximum file size (' . $filesize_limit . ') exeeded.');
//            $error = true;
        } else {
            $File_Name          = basename($file['name']);
            $File_Ext           = substr($File_Name, strrpos($File_Name, '.')); //get file extention
            $Random_Number      = rand(0, 9999999999); //Random number to be added to name.
            $NewFileName        = $Random_Number.$File_Ext; //new file name
            if (move_uploaded_file($file['tmp_name'], $UploadDirectory . $NewFileName)) {
                $files[] = $UploadDirectory . $file['name']; //$NewFileName; // 
                $finfo = finfo_open(FILEINFO_MIME_TYPE);
                $mime_type = finfo_file($finfo, $UploadDirectory . $NewFileName);
                //$data = array('Mime-Type (' . $mime_type . ')');
                finfo_close($finfo); 
                if ($mime_type != 'application/pdf') {
                    unlink($UploadDirectory . $NewFileName);
                    $data = array('File MUST be a PDF!');
                    $error = true;
                };
            } else {
                $error = true;
            }
        }
    }
    if ($error) {
        $data = array('error' => $data);
    } else {
        $data = array('files' => $files);
    }
} else {
    $data = array('success' => 'Form was submitted', 'formData' => $_REQUEST);
};
echo json_encode($data);

PHP错误日志

〔2016年5月17日00:59:49 UTC〕PHP注意：未定义的变量：userid在/home/deje/public_html/writers tryst/php/writers.php，第34行

〔2016年5月17日00:59:49 UTC〕PHP堆栈跟踪：

【2016年5月17日00:59:49 UTC】PHP 1。｛main｝（）/home/deje/public_html/writers tryst/php/writers.php：0

〔2016年5月17日00:59:49 UTC〕PHP致命错误：未捕获异常"异常"，消息为"您必须输入子类型。"在里面/home/deje/public_html/writers tryst/php/writers.php：49

堆栈跟踪：

在第49行/home/deje/public_html/writers-tryst/php/writers.php中抛出0｛main｝