上传图像iOS到PHP

我目前正试图从我的iOS应用程序上传到php后端图像。我在网上跟踪了样品;然而，我收到一个PHP错误代码3 (UPLOAD_ERR_PARTIAL)。有人能帮我吗?我正在使用[NSURLConnection sendAsynchronousRequest:queue:completionHandler].

我读过一些人禁用连接头，但这也不起作用。服务器端有什么我应该检查的吗?

<?
$uploaddir = '';      //Uploading to same directory as PHP file
$file = basename($_FILES['userfile']['name']);
$uploadfile = $uploaddir . $file;
if ($_FILES['userfile']['error'] === UPLOAD_ERR_OK) {
} else {
    die("Upload failed with error code " . $_FILES['userfile']['error']);
}
?>