为什么它在实时服务器上显示查询错误

Question

$sql = "INSERT INTO data
                    (id, docname, class, subject, docset, event, 
                     docobject, usermail) 
            VALUES ('$dname','$cls','$sub','$dset','$event',
                    '$target','$ssn')";
$result = mysqli_query($con,$sql);
if ($result==true) {
    if (move_uploaded_file($file, $target)) {
        echo "Data uploaded successfully.";
    } else {
        echo "Data upload not done properly!";
    }
} else {
    echo "Query error!";
}