if (isset($_SERVER['DOCUMENT_ROOT']) && !empty($_SERVER['DOCUMENT_ROOT'])) {
    $doc_root_path = $_SERVER['DOCUMENT_ROOT'];
    $resolved_doc_root_path = realpath($doc_root_path);

    if ($resolved_doc_root_path !== false) {
        if (is_dir($resolved_doc_root_path)) {
            if (is_readable($resolved_doc_root_path)) {
                $root_dir = $resolved_doc_root_path;
            } else {
                $error_message = "Error: The determined Document Root path exists but is not readable by the web server (check directory permissions): " . htmlspecialchars($doc_root_path, ENT_QUOTES);
            }
        } else {
            $error_message = "Error: The determined Document Root path is not a directory: " . htmlspecialchars($resolved_doc_root_path, ENT_QUOTES);
        }
    } else {
        $error_message = "Error: The Document Root path ('" . htmlspecialchars($doc_root_path, ENT_QUOTES) . "') reported by the server could not be found or accessed (check server configuration and directory permissions).";
    }
} else {
    $error_message = "Error: The web server configuration variable \$_SERVER['DOCUMENT_ROOT'] is not available or is empty. Cannot automatically determine the domain's homepage directory. Check your web server configuration (e.g., Apache VirtualHost or Nginx server block).";
}