无法在PHP中获得textarea值

Question

我有一个提交表单，用于在图像中编写标题。我为此使用了plupload。当文件上传时，它会附加一个div，里面是一个textarea，你可以在其中写下你的图片标题。但是在提交表单时我总是收到未定义索引的错误。 这是我上传文件时的代码：

uploader.bind('FileUploaded', function(up, file) {
    var filename = file.id + '.' + file.name.split(".").pop()
    var uploaded_files = $('#uploaded_files').attr('value');
    var appended_files = uploaded_files + file.id + ',';
    $('#uploaded_files').attr('value', appended_files);
    var prev = '<div class="col-sm-12" style="margin-bottom:5px;" id="upload-' + file.id + '">' +
            '<div class="col-sm-3">' +
            '<img class="img-thumbnail" width="100%" height="auto"  src="./assets/images/gallery/resorts_gallery/' + filename + '">' +
            '</div>' +
            '<div class="col-sm-6">' +
            '<textarea name="' + file.id + '_Caption" class="form-control" style="font-size:12px;height:70px" placeholder="Put a caption"></textarea>' +
            '</div>' +
            '<div class="col-sm-3"><a href="javascript:void(0);" onclick="remove_img(\'' + file.name + '\',\'' + file.id + '\')"><span class="btn btn-danger btn-xs"><span class="fa fa-times"></span></span></a></div>' +
            '</div>';
    $("#prevCap").append(prev);
});

在我的HTML中：

<form role="form" method="POST" id="submit-review" enctype="multipart/form-data">

    <div class="form-group">
        <label for="uploadphoto">Share your Experiences by uploading Photos</label> <span class="small-text">(optional)</span><br/>  
        <div class="well" id="caption-thumb"> <!-- preview and caption -->
            <div id="prevCap" class="col-sm-12" style="margin-bottom:10px"></div>
            <input type="hidden" name="uploaded_files" id="uploaded_files" value="" />
            <hr>
            <button class="btn btn-default navbar-btn" type="button" data-toggle="modal" data-target="#upload-modal">Add Photos</button>
            <hr id="hr-line" style="display:none">
        </div>
    </div>

    <input type="submit" class="btn btn-embossed btn-default" id="btnsubmit" name="submit" disabled value="SUBMIT"/>
</form>

在我的php中我做了这样的事情：

$last_id = insert_getID("reviews", $fields, $val);
$uploaded_files = $_POST['uploaded_files'];
$uploaded_files = explode(',', $uploaded_files);
$uploaded_files = array_filter($uploaded_files);
foreach ($uploaded_files as $file) {
    $wherefields = array('img_name', 'users_id');
    $upload_value = $file;
    $wherevalues = array($upload_value, $_SESSION['uid']);
    $where_file = where($wherefields, $wherevalues, "", "");
    $caption = $file . "_Caption";
    $file_caption = $_POST['\'' . $caption . '\''];
    $file_field = array('review_id', 'caption');
    $file_value = array($last_id, $file_caption);
    update('gallery', $file_field, $file_value, $where_file);
    echo "<pre>" + $file_caption + "</pre>";
}

当成功上传的文件时，textarea将附加在div #prevCap内，并且该div也在表单内。我收到错误是因为表单无法获得textarea的值。我该如何解决这个问题，如何获得附加的textarea值？