No PHP script should ever be able to crash the Apache process. If it does, it is a bug in PHP. Since you're using a somewhat exotic platform ;-) this is not unlikely. Try if there's an PHP upgrade for HPUX or compile a newer version of PHP yourself.
Yeah, I understand the statement "No <insert scripting language here> should be able to crash a <insert a non-related product here> process (with the caveat that it's running in a non-Windows environment)". The PHP we're using happens to be the one that's installed with apache (there is no other PHP on the machine - in fact, the only thing installed there other than the base OS is Apache). That leads to the other problem - I don't get to decide what gets put on that machine. It's a server that is more controlled
However, given that finding anything for HP-UX (particularly itanium version of HP-UX) in the "depot" format (analogue of "rpm" in RedHat world) is nigh-impossible, and I don't feel like downloading source and explaining to the admins how to compile software on that host, well, the fix of going back to an older version of GeSHi is by far the simpler version.