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?
Pretty much the only way to perform that integration (with PHP as the dominant language) is to shell out.  This means starting python manually every time you need it.  
That can be a little slow if you need to do it a lot.  You can mitigate this by creating the syntax hilite when posts are created or edited, not when viewing.
Using Pygments with PHP (Python in PHP)