PHP list of specific files in a directory

if ($handle = opendir('.')) {
    while (false !== ($file = readdir($handle)))
    {
        if ($file != "." && $file != ".." && strtolower(substr($file, strrpos($file, '.') + 1)) == 'xml')
        {
            $thelist .= '<li><a href="'.$file.'">'.$file.'</a></li>';
        }
    }
    closedir($handle);
}

A simple way to look at the extension using substr and strrpos

$it = new RegexIterator(new DirectoryIterator("."), "/\\.xml\$/i"));

foreach ($it as $filename) {
    //...
}

You can also use the recursive variants of the iterators to traverse an entire directory hierarchy.

You'll be wanting to use glob()

Example:

$files = glob('/path/to/dir/*.xml');