how do i link variables?

[shank888]

if($action == 'forum')
{
/// Shows each catagory
$query2="SELECT * FROM `cat`";
$result1=mysql_query($query2, $conn);
while ($row2=@mysql_fetch_array($result1))
	{
		@extract ($row2);
		echo"
		<table cellspacing='1' width='780'>
			<tr>
				<td colspan='4' width='100%'>
					<em>$cat_name</em>
				</td>
			</tr>
			<tr>
			<td width='50%'>
					<b>Forum</b>
				</td>
				<td width='10%'>
					<b>Topics</b>
				</td>
				<td width='10%'>
					<b>Replies</b>
				</td>
				<td width='30%'>
					<b>Most Recent Post</b>
				</td>
			</tr>";
/// Shows forums for each catagory
		$query3="SELECT * FROM `forum` WHERE forum_cat_id = $cat_id";
			$result2=mysql_query($query3,$conn);
			while ($row3=@mysql_fetch_array($result2))
				{
					@extract($row3);
					echo"
					<tr>
						<td width='50%'>
<a href='".$filename."?action=topic&cat_id=".urlencode("{$cat_id}")."&forum_id=".urlencode("{$forum_id}")."'>
						<b>$forum_name</b><br /></a>
							$forum_desc
						</td>
						<td width='10%'>
							$topic_id
						</td>
						<td width='10%'>
							$post_id
						</td>
						<td width='30%'>
							$recent_post
						</td>
					</tr>
					";
				}
		echo"</table>";
	}


///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
##Display Topics for selected forum Forum
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
if($action == 'topic') {
if ((!$_GET[cat_id]) || (!$_GET[forum_id])) {	


// Get topics

 

to get to the first page you would go http://URL.com/name.php?action=forum

 

but how do i get the link section:

 

"<a href='".$filename."?action=topic&cat_id=".urlencode("{$cat_id}")."&forum_id=".urlencode("{$forum_id}")."'>

<b>$forum_name</b><br /></a>

 

to go to 'topic'

 

[skali]

$_SERVER['QUERY_STRING']