List html files in directory for use with generating sitemap

<?php
   $dir = './';
   $dirHandle = opendir($dir);
   $returnstr = "";
   while ($file = readdir($dirHandle)) {
        if(!is_dir($file) && strpos($file, '.html')>0) {
           $returnstr .= $file;
      }
   }
   echo '<div align="center">';
   echo $returnstr;
   echo '</div>';
   closedir($dirHandle);
?>