无法使用下拉列表过滤记录

使用选定的字段catname过滤记录。需要使用类别过滤器过滤课程记录。类别ID选择正确。我需要按类别名称过滤。表类别中的下拉列表和课程表中要完成的过滤器。示例：如果$ selected = 1，则$ selectedcat ="信息技术"，然后将其传递给查询。这样它起作用。但是我需要直接使用类别过滤。

<?php
    include('header-basic-light.php');
    require_once('dbconnect.php');
    $selsql="SELECT catno,catname FROM category";
    $res=mysqli_query($con,$selsql);
    if ($res == '')
    {
        echo "No connection";
        die(mysqli_error($con));
    }
    $selected=0;
    if (isset($_POST['categories']))
    {
    $selected=$_POST['categories']; 
    //echo $selected;
    }

    $selectsql="SELECT * FROM courses";
    $res1=(mysqli_query($con,$selectsql));
    if (!mysqli_query($con,$selectsql)) 
    {
    die(mysqli_error($con));
    }
    if ($selected==1) 
    {
    $selectsql="SELECT * FROM courses where ccategory='$selectedcat'";
    $res1=(mysqli_query($con,$selectsql));
    }
    else
    if ($selected==2) 
    {
    $selectsql="SELECT * FROM courses where ccategory='$selectedcat'";
    $res1=(mysqli_query($con,$selectsql));
    }

    mysqli_close($con);
    ?>
    <html>
    <head>
        <title>Drop down list </title>
        <meta charset="UTF-8">
        <meta name="viewport">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
    </head>
    <body>
    <form action="<?php echo $_SERVER['PHP_SELF'];?>" method="post">
    <label for="categories">Select the Category : </label>
        <select name="categories" style="width:250px;" onchange="this.form.submit();">
    <?php 
    while($row=mysqli_fetch_assoc($res)) {
      if($selected==$row['catno']) {
        $selectedcat= $row['catname'];
        /* here you can get $selectedcat*/
      }
      ?>
      <option value="<?php echo $row['catno'];?>"
      <?php 
        if($selected == $row['catno']) { echo 'selected="selected"'; } 
      ?> 
      >
      <?php echo $row['catname'];?>
      </option>
    <?php } ?>
        </select>
     </form>
    <h2>View Information</h2>
    <table class="table">
    <tr>
    <th>#</th>
    <th>cname</th>
    <th>start_date</th>
    <th>duration</th>
    <th>Remarks</th>
    <th>Options</th>
    </tr>
    <?php
    while ($r=mysqli_fetch_assoc($res1))
    {
    ?>
    <tr>
    <td><?php echo $r['cno'];?></td>
    <td><?php echo $r['cname'];?></td>
    <td><?php echo $r['start_date'];?></td>
    <td><?php echo $r['duration'];?></td>
    <td><?php echo $r['remarks'];?></td>
    <?php if ($r['ccategory']=='Information Technology') {$catnum=1;}
    if ($r['ccategory']=='Management') {$catnum=2;} ?>
    <td><a href="loadpage.php?id=<?php echo $catnum;?>">Details&nbsp&nbsp</a>
    </tr>
    <?php } ?>
    </table>
    </body>
    </html>

请尝试此

<?php 
while($row=mysqli_fetch_assoc($res)) {
  if($selected==$row['catno']) {
    $selectedcat= $row['catname'];
    /* here you can get $selectedcat*/
  }
  ?>
  <option value="<?php echo $row['catno'];?>"
  <?php 
    if($selected == $row['catno']) { echo 'selected="selected"'; } 
  ?> 
  >
  <?php echo $row['catname'];?>
  </option>

，请参阅更新的答案，在任何mysqli指令之前，您都不应关闭mysqli连接。
因此，在页面底部或使用Code中的所有MySQLI指令

中使用mysqli_close($con);

<?php
   include('header-basic-light.php');
   require_once('dbconnect.php');
   $selsql="SELECT catno,catname FROM category";
   $res=mysqli_query($con,$selsql);
   if ($res == '')
   {
       echo "No connection";
       die(mysqli_error($con));
   }
$selected=0;
if (isset($_POST['categories']))
{
   $selected=$_POST['categories']; 
   //echo $selected;
}

$selectsql="SELECT * FROM courses";
$res1 = mysqli_query($con,$selectsql);
if (!mysqli_query($con,$selectsql)) 
{
   die(mysqli_error($con));
}
if ($selected != 0) 
{
  /* fetch $selectedcat  from input hidden name hid_selected_cat_name*/
   $selectedcat = mysqli_real_escape_string($con,trim($_POST['hid_selected_cat_name'])); 
   $selectsql="SELECT * FROM courses where ccategory='$selectedcat'";
   $res1=mysqli_query($con,$selectsql);
}

?>
<html>
<head>
    <title>Drop down list </title>
    <meta charset="UTF-8">
    <meta name="viewport">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
</head>
<body>
<form action="<?php echo $_SERVER['PHP_SELF'];?>" method="post">
<label for="categories">Select the Category : </label>
    <select name="categories" style="width:250px;" onchange="this.form.submit();">
<?php 
while($row=mysqli_fetch_assoc($res)) {
  if($selected==$row['catno']) {
    $selectedcat= $row['catname'];
    /* here you can get $selectedcat*/
  }
  ?>
  <option value="<?php echo $row['catno'];?>"
  <?php 
    if($selected == $row['catno']) { echo 'selected="selected"'; } 
  ?> 
  >
  <?php echo $row['catname'];?>
  </option>
<?php } ?>
    </select>
    /* store $selectedcat into hidden id*/
    <input type="hidden" id="hid_selected_cat_name" name="hid_selected_cat_name" value="<?php echo $selectedcat;?>" >
 </form>
<h2>View Information</h2>
<table class="table">
<tr>
<th>#</th>
<th>cname</th>
<th>start_date</th>
<th>duration</th>
<th>Remarks</th>
<th>Options</th>
</tr>
<?php
while ($r=mysqli_fetch_assoc($res1))
{
?>
<tr>
<td><?php echo $r['cno'];?></td>
<td><?php echo $r['cname'];?></td>
<td><?php echo $r['start_date'];?></td>
<td><?php echo $r['duration'];?></td>
<td><?php echo $r['remarks'];?></td>
<?php if ($r['ccategory']=='Information Technology') {$catnum=1;}
if ($r['ccategory']=='Management') {$catnum=2;} ?>
<td><a href="loadpage.php?id=<?php echo $catnum;?>">Details&nbsp&nbsp</a>
</tr>
<?php } ?>
</table>
</body>
</html>

简单的一个。尝试一下。

<?php    
 while($row=mysqli_fetch_assoc($res)) {
 $selectedFlag = ($selected==$row['catno']) ? "selected" : "";
?>
<option value="<?php echo $row['catno'];?>" <?php echo $selectedFlag ?> >  
  <?php echo $selectedcat= $row['catname'];?>
</option> 
<?php } ?>

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