function getHtmlRootFolder(string $root = '/var/www/') {

    // -- try to use DOCUMENT_ROOT first --
    $ret = str_replace(' ', '', $_SERVER['DOCUMENT_ROOT']);
    $ret = rtrim($ret, '/') . '/';

    // -- if doesn't contain root path, find using this file's loc. path --
    if (!preg_match("#".$root."#", $ret)) {
        $root = rtrim($root, '/') . '/';
        $root_arr = explode("/", $root);
        $pwd_arr = explode("/", getcwd());
        $ret = $root . $pwd_arr[count($root_arr) - 1];
    }

    return (preg_match("#".$root."#", $ret)) ? rtrim($ret, '/') . '/' : null;
}