PHP Non Recursive Function Through Directories

I needed a recursive function to go through directories and subdirectories, so I headed to PHP.net's readdir() page, knowing that I would find one in user comments. Or did I need a recursive function ? Hell no. I've found the nifty neat following function, which does not recursively calls itself. And what's so cool about it, you may ask ? Speed.

<?php
function list_directory($dir) {
   $file_list = '';
   $stack[] = $dir;
   while ($stack) {
      $current_dir = array_pop($stack);
      if ($dh = opendir($current_dir)) {
         while (($file = readdir($dh)) !== false) {
            if ($file !== '.' AND $file !== '..') {
               $current_file = "{$current_dir}/{$file}";
               if (is_file($current_file)) {
                  $file_list[] = "{$current_dir}/{$file}";
               } elseif (is_dir($current_file)) {
                  $stack[] = $current_file;
               }
            }
         }
      }
   }
   return $file_list;
}
?>

The function returns an array of files from directory passed as argument and subdirectories. The user who submitted this function added a few benchmarks against an average recursive function, to find out that this one is about 50% quicker.

Just as anybody I guess, I've been using various recursive functions for years now, and I just had never thought about why or how to do it differently. Damn. So many CPU cycles wasted for nothing :)