我是php的初学者。我想从选项中选择部门，选择部门后，我想在下一个下拉框中显示属于该部门的卷号。通过提供一些与我的问题相关的想法来帮助我。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title>Untitled Document</title>
</head>
<body>
<?php include('C:'wamp'www'fms'background.php'); ?>
<?php include('C:'wamp'www'fms'adminmenu.php'); ?>
<?php
if (isset($_POST['delete'])) {
$con = mysql_connect("localhost", "root", "");
if (!$con) {
die('Could not connect: ' . mysql_error());
}
$cid = $_POST['cid'];
$sql = "DELETE from addpassenger WHERE rno='$rno'";
mysql_select_db('fms');
$retval = mysql_query($sql, $con);
if (!$retval) {
die('Could not delete data: ' . mysql_error());
}
echo "Deleted data successfully'n";
mysql_close($con);
} else {
?>
<p></p>
<p></p>
<p></p>
<p></p>
<center>
<form method="post">
<table width="344" border="0" cellspacing="1" cellpadding="2">
<tr>
<td>SELECT THE DEPARTMENT</td>
<td><?php
$con = mysql_connect("localhost", "root", "") or die(mysql_error());
$db = @mysql_select_db("fms", $con) or die(mysql_error());
$str = "select dept from addpassenger";
$res1 = @mysql_query($str);
echo '<select name="dept">';
echo '<option selected="----------"></option>';
while ($row = mysql_fetch_array($res1)) {
echo '<option value="' . $row['dept'] . '">' . $row['dept'] . '</option>';
}
echo '</select>';
?></td>
</tr>
<tr>
<td width="208">SELECT THE ROLL NO</td>
<td width="125">
<?php
$con = mysql_connect("localhost", "root", "") or die(mysql_error());
$db = @mysql_select_db("fms", $con) or die(mysql_error());
$str = "select rno from addpassenger ";
$res1 = @mysql_query($str);
echo '<select name="rno">';
echo '<option selected="----------"></option>';
while ($row = mysql_fetch_array($res1)) {
echo '<option value="' . $row['rno'] . '">' . $row['rno'] . '</option>';
}
echo '</select>';
?>
</select>
</td>
</tr>
<tr>
<td width="208"></td>
<td></td>
</tr>
<tr>
<td width="208"></td>
<td>
<input name="delete" type="submit" id="delete" value="Delete">
</td>
</tr>
</table>
</form>
</center>
<?php
}
?>
</body>
</html>
select rno from addpassenger
需要有一个where子句，该子句根据以前的选择