Loop thumbnails and display - php

Question

I'm trying to loop images from a thumbnails folder (with already presized thumbnails) and link them to their respective full size images. The names will match exactly so I should be able to do this with one loop.
The question is do I need to open both directories before I can start looping through them? In my example, I'm pulling from the Montague Vacations folder which has both the thumbs and full size folders.
$files = array();
$dir = opendir('./assets/vacations/montague'); // open the cwd..also do an err chec
while(false != ($file = readdir($dir))) {
if(($file != ".") and ($file != "..") and ($file != "index.php")){
$files[] = $file; // put in array.
}
}
natsort($files); // sort.
$counter = 0;
// print.
foreach($files as $file) {
$counter++;
echo "<code>";
print htmlentities("
<li>
<!--[if gte IE 9]><!--><a href='#pic-$counter'><!--<![endif]-->
<!--[if lte IE 8]><a href=/userfiles/images/portfolio/web-templates/big/fastedit.png' target='_blank'><![endif]-->
<img src='%base_url%/assets/vacations/montague/thumb/$file alt='' />
<span><b>Preview</b></span>
</a>
<div id='pic-$counter' class='overlay'>
<img src='%base_url%/assets/vacations/montague/full/$file' alt='' />
<div>
<p>Picture caption here.</p>
</div>
<a href='#close' class='close' title='Close'>×</a>
</div>
</li>
");
echo ("<br />\n");
echo ("<br />\n");
echo "</code>";
}