Generate MD5 hash for directory

/**
 * Generate an MD5 hash string from the contents of a directory.
 *
 * @param string $directory
 * @return boolean|string
 */
function hashDirectory($directory)
{
    if (! is_dir($directory))
    {
        return false;
    }

    $files = array();
    $dir = dir($directory);

    while (false !== ($file = $dir->read()))
    {
        if ($file != '.' and $file != '..')
        {
            if (is_dir($directory . '/' . $file))
            {
                $files[] = hashDirectory($directory . '/' . $file);
            }
            else
            {
                $files[] = md5_file($directory . '/' . $file);
            }
        }
    }

    $dir->close();

    return md5(implode('', $files));
}