script / class to parse stackoverflow-like markup

I'm looking for a script that parses markup like the one used in wikis and in stackoverflow...

I prefer something that I can easily customize for additional markup

I'm using PHP, and I'm looking for a standalone class that requires no external dependencies from some framework, lib, etc.

I'm talking about a class/script that converts this:

**bold** 
- aaa
- bbb
- ccc

into:

bold

• aaa
• bbb
• ccc

The specific markup language used by StackOverflow is called Markdown. A Google search for PHP Markdown turns up several libraries; I haven't used any of them so I can't recommend one.