gistfile1.php

<?php


$content = get_the_content();

$nodes = extract_tags($content, 'h\d+', false);

$headingsList = "";

$currentHeading = 0;
$previousHeading = 0;

$level = 0;

if($instance['level'])
        $level = substr($instance['level'], 1, 2);

if($level){
        foreach ($nodes as $key=>$node) {
                if (substr($node['tag_name'], 1, 2) > $level)
                        unset($nodes[$key]);
        }
}

foreach ($nodes as $node) {

        $previousHeading = $currentHeading;
        $currentHeading = substr($node['tag_name'], 1, 2);

        if ($currentHeading < $previousHeading) {
                for($i=0 ; $i<$previousHeading-$currentHeading ; $i++){
                        $headingsList .= '</ul>';
                }
        }

        if ($currentHeading > $previousHeading) {
                $headingsList .= '<ul>';
        }

        $headingsList .= '<li><a href="#';
        $headingsList .= str_replace(" ", "_", strip_tags($node['contents']));
        $headingsList .= '">';
        $headingsList .= substr(strip_tags($node['contents']),0,$instance['hchars']);
        $headingsList .= '</a></li>';
}

if (count($nodes) >= 1){
        echo $before_widget;
        if (strlen($instance['title']) > 0) {
                echo $before_title.$instance['title'].$after_title;
        }
        echo $headingsList;
        echo $after_widget;
}
?>