Automatically generate nested table of contents based on heading tags

Question

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