Is it possible to use Python (specifically Pygments) with PHP? Currently, I have a phpBB forum that I'm developing for and JS Syntax Highlighters just haven't been working for me. There's already a GeSHI mod, but I want to develop something myself just for experience.
Also, would there be performance issues?
If you're interested in diving into Python, you could write an external script or server application to update new posts with syntax-highlighted code.  If it were me, I'd retain the original code in one database column and place the syntax-highlighted version in another.
A simple script to update new posts in batches could run as a cron job at whatever interval you find ideal.
To support a near real-time scenario, you could write a server application that sits and waits to be notified of new posts one at a time.  For example, upon processing a new post, the PHP application could send the highlighting application a message through an AMQP queue.
Using Pygments with PHP (Python in PHP)