# Displaying topics in a PHP forum

I am very new to PHP. I've only been using it for a few months. I'm currently attempting to build a forum and wanted feedback on some of the code I have so far. These are two files I feel like could have been done better. There are no errors as far as I can tell. I would just like some feedback on security issues, coding practices, etc.

File 1:

<?php

function dispcategories(){
include ('../db.php');
$select = mysqli_query($mysqli, "SELECT * FROM categories");

while($row = mysqli_fetch_assoc($select)){
echo "<table class = 'category-table'";
echo "<tr><td class= 'main-category'colspan='2'>".$row['category_title']."</tr></td>"; dispsubcategories($row['cat_id']);
echo "</table>";
}
}

function dispsubcategories($parent_id){ include ('../db.php');$select = mysqli_query($mysqli, "SELECT cat_id, subcat_id, subcategory_title, subcategory_descr FROM categories, subcategories WHERE ($parent_id = categories.cat_id) AND ($parent_id = subcategories.parent_id)"); echo "<tr><th width='90%'>Categories</th><th width='10%'>Topics</th></tr>"; while($row = mysqli_fetch_assoc($select)){ echo "<tr><td class='category_title'><a href='topics.php/".$row['cat_id']."/".$row['subcat_id']."'> ".$row['subcategory_title']."<br/>";
echo $row['subcategory_descr']."</a></td>"; echo "<td class='num-topics'>".getnumtopics($parent_id, $row['subcat_id'])."</td></tr>"; } } //Displays categories function getnumtopics($cat_id, $subcat_id){ include ('../db.php');$select = mysqli_query($mysqli, "SELECT category_id, subcategory_id FROM topics WHERE ".$cat_id." = category_id
AND ".$subcat_id." = subcategory_id");$get = mysqli_num_rows($select); return$get;
}

//Displays Topics Within categories
function disptopics($cid,$scid){
include ('../db.php');
$select = mysqli_query($mysqli, "SELECT topic_id, author, title, date_posted, views, replies FROM categories, subcategories, topics
WHERE ($cid = topics.category_id) AND ($scid = topics.subcategory_id) AND ($cid = categories.cat_id) AND ($scid = subcategories.subcat_id) ORDER BY topic_id DESC");

if(mysqli_num_rows($select) != 0) { echo "<table class='topic-table'>"; echo "<tr><th>Title</th><th>Posted By</th><th>Date Posted</th><th>Views</th><th>Replies</th></tr>"; while($row = mysqli_fetch_assoc($select)){ echo "<tr><td><a href='readtopic.php/".$cid."/".$scid."/".$row['topic_id']."'>
".$row['title']."</a></td><td>".$row['author']."</td><td>".$row['date-posted'].".</td><td>".$row['views']."</td>
<td>".$row['replies']."</td></tr>"; } echo "</table>"; } else { echo "<p>This category has no topics yet! <a href='newtopic.php/".$cid."/".$scid."'> Add a new one!</a></p>"; } } ?>  File 2:  <?php /*$select = mysqli_query($mysqli, "SELECT * FROM categories"); while($row = mysqli_fetch_assoc($select)){ echo "<h1>",$row['category_title']."</h1>" ;
}*/

//disptopics($_GET['cid'],$_GET['scid']);
if (isset($_GET['cid'],$GET['scid'])){

disptopics();
}

?>