当我显示此页面时，代码的PHP部分不起作用.它不会显示主题

代码不会显示数据库中的主题。我只是得到一个空白页。
有什么解决办法吗？
页面加载，但不会显示任何内容。他们希望我添加更多上下文，但它破坏了它，所以你去吧。

<?php
//Database stuff.
include_once("connect.php");
if ($conn->connect_error) {
    trigger_error('Database connection failed: '  . $conn->connect_error,   E_USER_ERROR);
}else{
    mysqli_select_db($conn,"2159928_db");
}
$tid = '';
$cid = $_GET['cid'];
$tid = $_GET['tid'];
$sql = "SELECT * FROM topics WHERE category_id='".$cid."' AND     id='".$tid."' LIMIT 1";

$result = mysqli_query($conn, $sql);
if (mysqli_num_rows($result) == 1){
  echo "<table width ='75%'>";
  if (isset($_SESSION['uid'])) {
    //echo "<form action ='post_reply.php?cid=".$cid." &tid =".$tid. "'   method = 'post'>
    //<input type = 'submit' name = 'submit' value = 'Reply'/>";
    //echo "<tr><td colspan ='2'><center><input   type='submit'              value='Reply' onClick = 'window.open = 'post_reply.php?cid=".$cid." &tid =".$tid."' />";
    echo "<tr><td colspan ='2'><center><input type='submit'  value='Reply' onClick='window.open('"post_reply.php?cid=$cid&tid=$tid'")'  />";
  } else {
    echo "<tr><td colspan = '2'><p><center> Please login to reply to  topics.</p></td></tr>";
  }  
   //Trying to display this. Doesn't even display border box.
   while ($row = mysqli_fetch_assoc($result)) {
       $sql2 = "SELECT * FROM posts WHERE category_id='".$cid."' AND  topic_id='".$tid. "'";
       $result2 = mysqli_query($conn, $sql2);       
       while ($row2 = mysqli_fetch_assoc($result2)){
           echo "<tr><td valign ='top' style = 'border: 5px solid  #ffffff;'><div style = 'min-height: 125px;'>".$row['topic_title']."<br/>
        by ".$row2['post_username']. " - " .$row2['post_date']. "<hr/>".$row2['post_content']."</div></td>";
       }
       //This part not relevant.
       $old_views = $row['topic_views'];
       $new_views = $old_views + 1;
       $sql3 = "UPDATE topics SET topic_views='".$new_views."' WHERE  category_id='".$cid."' AND id ='".$tid."' LIMIT 1";
    $result3 = mysqli_query($conn, $sql3);
  }
    echo "</table>";
} else {
    echo "<p>This topic does not exist.</p>";
}
?>