Is there a php version of the markup used for the stackexchange, more info on the mark up at Stackoverflow's Markup.
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
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The most obvious is PHP Markdown.
That said, this is based on the original Markdown project by John Gruber, so won't necessarily contain the various improvements made by the SO team.
Whilst not directly relevant, there are some good blog posts and podcasts regarding the history and specific implementation of markdown on SO:
(Remembers how much he enjoyed the podcasts. Becomes momentarily maudlin. Etc.) ;-)