Automatically generate nested table of contents based on heading tags

Which one of you crafty programmers can show me an elegant php coded solution for automatically generating a nested table of contents based on heading tags on the page?

So I have a html document thus:

<h1> Animals </h1>

Some content goes here.
Some content goes here.

<h2> Mammals </h2>

Some content goes here.
Some content goes here.

<h3> Terrestrial Mammals </h3>
Some content goes here.
Some content goes here.

<h3> Marine Mammals </h3>
Some content goes here.
Some content goes here.

<h4> Whales </h4>
Some content goes here.
Some content goes here.

More specifically, I want a linked table of contents in the form of a nested list of links to headings on the same page:

Table of Contents (automatically generated by PHP code)

1. Animals
   1. Mammals
      1. Terrestrial_Mammals
      2. Marine_Mammals
         1. Whales

I found this method, by Alex Freeman (http://www.10stripe.com/articles/automatically-generate-table-of-contents-php.php):

    preg_match_all('#<h[4-6]*[^>]*>.*?<\/h[4-6]>#',$html_string,$resultats);

    //reformat the results to be more usable
    $toc = implode("\n",$resultats[0]);
    $toc = str_replace('<a name="','<a href="#',$toc);
    $toc = str_replace('</a>','',$toc);
    $toc = preg_replace('#<h([4-6])>#','<li class="toc$1">',$toc);
    $toc = preg_replace('#<\/h[4-6]>#','</a></li>',$toc);

    //plug the results into appropriate HTML tags
    $toc = '<div id="toc"> 
    <p id="toc-header">Table des matières</p>
    <hr />
    <ul>
    '.$toc.'
    </ul>
    </div><br /><br />';

    return $toc;

In the HTML, the headers have to be written as:

<h2><a name="target"></a>Text</h2>