我的代码显示错误，我不知道为什么。
我得到的错误是：
警告：无法修改标头信息-标头已由发送 （输出从C：\ xampp \ htdocs \ addnewstudent_insert.php：92开始） 第107行的C：\ xampp \ htdocs \ addnewstudent_insert.php失败：数据库 上传时遇到错误！！！
我的代码：
<html lang="en">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="icon" href="images/favicon.jpg">
<title>Library</title>
<!-- Bootstrap core CSS -->
<link href="css/bootstrap.min.css" rel="stylesheet">
<link href="css/ie10-viewport-bug-workaround.css" rel="stylesheet">
<link href="starter-template.css" rel="stylesheet">
<script src="js/ie-emulation-modes-warning.js"></script>
<link href="css/style.css" rel="stylesheet">
<script type="text/javascript">
function Validate()
{
var password = document.getElementById("p1").value;
var confirmPassword = document.getElementById("p2").value;
if (password != confirmPassword)
{
alert("Passwords do not match.");
return false;
}
return true;
}
</script>
</head>
<body>
<nav class="navbar navbar-default">
<div class="container">
<div class="navbar-header">
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#">Library</a>
</div>
<div id="navbar" class="collapse navbar-collapse">
<ul class="nav navbar-nav">
<li class="active"><a href="dashboard_staff.php">Dashboard</a></li>
<li><a href="help.php">Help</a></li>
<li><a href="contact.php">Contact</a></li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li><a href="#">Welcome to Staff Dashboard</a></li>
<li><a href="logout.php">Logout</a></li>
</ul>
</div><!--/.nav-collapse -->
</div>
</nav>
<header id="header">
<div class="container">
<div class="row">
<div class="col-md-10">
<h1><span class="glyphicon glyphicon-book" aria-hidden="true"></span> Library | Dashboard <small>Manage your books here</small></h1>
</div>
</div>
</div>
</header>
<section id="breadcrumb">
<div class="container">
<ol class="breadcrumb">
<li class="active">You are inside the staff dashboard of library management system</li>
</ol>
</div>
</section>
<section id="main">
<div class="container">
<div class="row">
<div class="col-md-3">
<div class="list-group">
<a href="dashboard_staff.php" class="list-group-item">
<span class="glyphicon glyphicon-cog" aria-hidden="true"></span> Dashboard
</a>
<a href="viewbook_staff.php" class="list-group-item "><span class="glyphicon glyphicon-eye-open" aria-hidden="true"></span> View Books</a>
<a href="addnewbook.php" class="list-group-item"><span class="glyphicon glyphicon-new-window" aria-hidden="true"></span> Add New Book</a>
<a href="deletebook.php" class="list-group-item"><span class="glyphicon glyphicon-erase" aria-hidden="true"></span> Delete a Book</a>
<a href="issue.php" class="list-group-item"><span class="glyphicon glyphicon-file" aria-hidden="true"></span> Issue and Return</a>
<a href="addnewstudent.php" class="list-group-item active main-color-bg"><span class="glyphicon glyphicon-plus" aria-hidden="true"></span> Add a Student</a>
<a href="viewstudent.php" class="list-group-item"><span class="glyphicon glyphicon-user" aria-hidden="true"></span> View Student</a>
</div>
</div>
<div class="col-md-9">
<div class="panel panel-default">
<div class="panel-heading main-color-bg">
<h3 class="panel-title">Dashboard</h3>
</div>
<div class="panel-body">
<?php
require_once("dbConnect.php");
$regno=$_POST['regno'];
$name=$_POST['stu_name'];
$class=$_POST['class'];
$stu_user=$_POST['stu_username'];
$stu_password=$_POST['stu_password1'];
$query="INSERT INTO login_student VALUES (DEFAULT,'$regno','$name','$class','$stu_user', '$stu_password');";
$result=mysqli_query($con,$query);
if ($result)
{
header( "refresh:3;url=addnewstudent.php" );
echo "New Student Added Successfully!";
}
else {
header( "refresh:3;url=addnewstudent.php" );
echo "Failed:Database has encountered an error while uploading!!!";
}
mysqli_close($con);
?>
</div>
</div>
</div>
</div>
</div>
</section>
</body>
</html>