How can I get the threadid?

[LanciaStratos]

I'm trying to change the URL's for forums and threads in an effort to make them more friendly to search engines like Google. I know there are several hacks with a rather different way of doing this, but I'm giving this method a shot. Right now I'm working on displaying threads by changing the normal URL (http://www.domain.com/forum/showthread.php?s=&threadid=123) to the new one (http://www.domain.com/forum/thread/123). I've almost got it working - here's what I've done so far:

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1. Created a copy of showthread.php and named the new file simply "thread" without any extension.

2. I then uploaded a .htaccess file in my forum directory containing the following (which will cause Apache to read the new "thread" file as a PHP script).

<Files thread> 
ForceType application/x-httpd-php 
</Files>

3. And, finally, in this new "thread" file, I've added these two lines (which I found from a post on the vB.com forums, they are highlighted in orange) at the very beginning of the file.

<?php

error_reporting(7);

$url_array=explode("/",$PATH_INFO); 
$threadid=$url_array;
$action = $HTTP_GET_VARS['action'];
$goto = $HTTP_GET_VARS['goto'];

if ( isset($goto) and ($goto=='lastpost' or $goto=='newpost')) {
  $noheader=1;
}

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After I perform the modifications above, I am able view a thread by using the new URL (http://www.domain.com/forum/thread/123). However, it will *only* show me the first thread on the forum (with a threadid of 1). I know the problem lies in the $threadid=$url_array; line, but I don't know what to change it to to fix it so that it will display whatever threadid number I specify in the URL. How should I change that line to achieve the desired effect?