There are markdown parsers available for PHP (http://michelf.ca/projects/php-markdown/ as an example), but the websocket thing might be quite challenging. PHP's architecture doesn't lend itself well for many concurrent long-running connections.

Unless you want to run out of RAM very quickly, you are much better off to do everything involving long-running connections using node.js or any other environment that works in an event based fashion as threads (and in case of most safe PHP architectures, usually processes) are way too expensive for that.