$content = get_the_content();

                    $dom = new DomDocument();
                    $dom->loadHTML('<?xml encoding="utf-8" ?>' . $content);

                    $xpath = new DOMXPath($dom);
                    $heads = $xpath->query('//h1|//h2|//h3|//h4|//h5|//h6');

                    echo '<ul>';
                    foreach ($heads as $tag) {
                        echo '<li><a href="#'. sanitize_title($tag->nodeValue) .'" title="'. $tag->nodeValue .'">'. $tag->nodeValue .'</a></li>';
                    }
                    echo '</ul>';