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我是PHP编程的新手，我在使用下拉列表模块。php dropdownlist未定义的索引
遇到PHP错误严重性：通知消息：未定义索引：yos。
这是代码。我希望有人帮助我。
<?php
$servername="localhost";
$username="root";
$password="";
$dbname="online_voting_web";
//Create connection
$con=mysqli_connect($servername,$username,$password,$dbname);
$name = $_POST['firstname'];
$name2 = $_POST['lastname'];
$name3 = $_POST['middlename'];
$username = $_POST['username'];
$pass = md5($_POST['password']);
$yos = isset($_POST['yos']) ? $_POST['yos'] : '';
$id = $_POST['stud_id'];
if(!$name || !$name2 ||$name3 || $id || !$year || !$pass|| !$username){
$error="Please fill empty fields";
include"student_reg.php";
exit();
}
$sql = "SELECT stud_id FROM stud_name WHERE stud_id='$id'";
$query = mysqli_query($con,$sql)or die(mysqli_error($con));
$exist = mysqli_num_rows($query);
if($exist==1){
$nam="<center><h4><font color='#FF0000'>The student ID already exist,register another</h4></center></font>";
unset($id);
include('student_reg.php');
exit();
}
$sql = "SELECT stud_username FROM stud_user WHERE username='".$_POST['username']."'";
$query = mysqli_query($con,$sql)or die(mysqli_error($con));
$exist = mysqli_num_rows($query);
if($exist==1){
$nam="<center><h4><font color='#FF0000'>The student ID already exist,register another</h4></center></font>";
unset($id);
include('student_reg.php');
exit();
}
$sql = "INSERT INTO stud_name(stud_id,studFname,stud_Lname,stud_Mname)
VALUES('$id','$name','$name2','$name3')";
$sql2 = "INSERT INTO stud_course(stud_username,stud_year)
VALUES('$username','$year')";
$sql3 = "INSERT INTO stud_user(stud_id,stud_username)
VALUES('$id','$username')";
$sql4 = "INSERT INTO login_(username,password)
VALUES('$username','$pass')";
$result = mysqli_query($con,$sql);
$result2 = mysqli_query($con,$sql2);
$result3 = mysqli_query($con,$sql3);
$result4 = mysqli_query($con,$sql4);
if(!$result && !$result2 && !$result3 &&!$result4){
die("Error on mysql query".mysqli_error());
}
else{
header("location: index.php");
}
?>
这是代码的从另一种形式的一部分，我第一次使用这个词YOS
<!--Style link-->
<link href="layout/styles/layout.css" rel="stylesheet" type="text/css">
<!--Navigation-->
</div>
</div>
<body id="top">
<div class="wrapper row3">
<div class="rounded">
<main class="container clear">
<div class="group btmspace-30">
<div id="register">
<!--End of Navigation-->
<fieldset>
<legend><h3>Student Registration Form</h3></legend>
<h4 style="color:#e60808;">
<?php global $nam; echo $nam;?> </h4>
<?php global $error; echo $error;?>
<!-- the login form-->
<form enctype="application/x-www-form-urlencoded" action="" method="post" id="myform">
<div class="register">
<table width="400px">
<tr><td><label>Firstname</label></td>
<td><input type="text" name="firstname" value="" style="width:250px; height:34px; border:1px solid #336666;"/></td>
</tr>
<tr><td><label>Middlename</label></td>
<td><input type="text" name="middlename" value="" style="width:250px; height:34px; border:1px solid #336666;"/></td>
<tr><td><label>Lastname</label></td>
<td><input type="text" name="lastname" value="" style="width:250px; height:34px; border:1px solid #336666;"/></td>
</tr>
<tr><td><label>Username</label></td>
<td><input type="text" name="username" value="" style="width:250px; height:34px; border:1px solid #336666;"/></td>
</tr>
<tr><td><label>Password</label></td>
<td><input type="password" name="password" value="" style="width:250px; height:34px; border:1px solid #336666;"/></td>
</tr>
<tr>
<td><label>Year Level</label></td>
<td><select name="yos" id="yos" style="width:250px; height:34px; border:1px solid #336666;">
<option value="000">Choose Level</option>
<option value="G7">Grade 7</option>
<option value="G8">Grade 8</option>
<option value="G9">Grade 9</option>
<option value="G10">Grade 10</option>
<option value="G11a">Grade 11-GAS</option>
<option value="G11b">Grade 11-Blhablha</option>
<option value="G12a">Grade 12-Blahblah</option>
<option value="G12b">Grade 12-khfakhf</option>
</select></td>
</tr>
<tr><td><label>ID Number</label></td>
<td><input type="text" name="stud_id" value="" style="width:250px; height:34px; border:1px solid #336666;"/></td>
</tr>
<tr><td colspan="2" align="center"><input type="submit" name="submit" value="Submit" style="width:100px; height:34px; border:1px solid #336666; border-radius:4px;"></td></tr>
</table>
</div>
</form>
<script type="text/javascript">
var frmvalidator = new Validator("myform");
frmvalidator.addValidation("firstname","req","Please enter student firstname");
frmvalidator.addValidation("firstname","maxlen=50");
frmvalidator.addValidation("middlename","req","Please enter student middlename");
frmvalidator.addValidation("middlename","maxlen=50");
frmvalidator.addValidation("lastname","req","Please enter student middlename");
frmvalidator.addValidation("lastname","maxlen=50");
frmvalidator.addValidation("username","req","Please enter student username");
frmvalidator.addValidation("username","maxlen=50");
frmvalidator.addValidation("password","req","Please enter student password");
frmvalidator.addValidation("password","minlen=6","Password must not be less than 6 characters.");
frmvalidator.addValidation("year","req","Please enter student year");
frmvalidator.addValidation("year","dontselect=000","You don't select year");
frmvalidator.addValidation("stud_id","req","Please enter your ID number");
frmvalidator.addValidation("stud_id","maxlen=50");
</fieldset>
</div>
center，font？ 20世纪90年代最后的HTML返回 – nogad
一些合理的代码缩进将是一个好主意。它可以帮助我们阅读代码，更重要的是，它可以帮助您**调试您的代码** [快速浏览编码标准]（http://www.php-fig.org/psr/psr-2/ ）为了您自己的利益。您可能会被要求在几周/几个月内修改此代码 ，最后您会感谢我。 – RiggsFolly
你的脚本存在[SQL注入攻击]的风险（http://stackoverflow.com/questions/60174/how-can-i-prevent-sql-injection-in-php） 看看发生了什么[小鲍比表]（http://bobby-tables.com/）即使 [如果你是逃避投入，它不安全！]（http://stackoverflow.com/questions/5741187/sql-injection-that-gets -around-mysql-real-escape-string） 使用[prepared parameterized statements]（http://php.net/manual/en/mysqli.quickstart.prepared-statements.php） – RiggsFolly