//首先，取得浏览器的传值：news_title和news_class
if(isset($_GET['news_class']))
{
  $news_class=$_GET['news_class'];
}
else
{
  $news_class='';
}
 
if(isset($_GET['news_title']))
{
  $news_title=$_GET['news_title'];
}
else
{
  $news_title='';
}

//然后开始判断，准备显示有关的内容

if ($news_title='')//当news_title元素为空时，开始判断news_class元素是否为空
{
   if ($news_class='')//news_class也为空，则该页面显示所有news表的news_title列表
    {
      $sql="SELECT news_title,in_time FROM news";
      $result = mysql_query($sql);
       while ($row=mysql_fetch_assoc($result))
             {
?>             
        <tr>
          <td>
        <?php echo $row['news_title']; ?>
        <?php echo $row['in_time']; ?>
      </td>
        </tr>
<?php
             }
mysql_free_result($result);
    }
    else//当news_class不为空，则该页面显示news_class的相关内容
    {
      $sql="SELECT * FROM news where news_class='$news_class'";
      $result = mysql_query($sql);
       while ($row=mysql_fetch_assoc($result))
             {
?>             
        <tr>
          <td><?php echo $row['news_title']; ?>
              <?php echo $row['in_time']; ?>
          </td>
        </tr>
<?php
             }
mysql_free_result($result);
    }
}
else  //当news_title不为空值时，开始判断news_class是否为空
{
   if ($news_class='')//news_class为空，则该页面show出news_title的相关数据
    {
      $sql="SELECT * FROM news where news_title='$news_title'";
      $result = mysql_query($sql);
       while ($row=mysql_fetch_assoc($result))
             {
?>             
        <tr>
          <td> <?php echo $row['news_title']; ?>
               Author : <?php echo $row['news_author']; ?> 
               Date : <?php echo $row['in_time']; ?> 
                      <?php echo $row['news_content']; ?>
      </td>
        </tr>
<?php
             }
mysql_free_result($result);      
    }
    else//当news_class也不为空值时，则该页面显示所有news表的数据（和两者都为空时一样）
    {
      $sql="SELECT news_title,in_time FROM news";
      $result = mysql_query($sql);
       while ($row=mysql_fetch_assoc($result))
             {
?>             
        <tr>
          <td><?php echo $row['news_title']; ?>
          <?php echo $row['in_time']; ?>
      </td>
        </tr>
<?php
             }
mysql_free_result($result);      
    }
}

if ($a && $b) {
    // 显示全部
} elseif ($a) {
    ;
} elseif ($b) {
    ;
}

------解决方案--------------------
url 要写作 index.php?news_title=xxx&news_class=xxx

$news_title = $news_class = '';
if(isset($_GET['news_title'])) $news_title = $_GET['news_title'];
if(isset($_GET['news_class'])) $news_class = $_GET['news_class'];

if($news_title != '' && $news_class != '') {
 //两个都有