We've just bought a new Power Mac G5 Macintosh computer for the office and
I'll need to do some backend engineering work using it. I've never worked
with PHP on MAC, so I really have no experience with what happens on a
MAC... are there on issues regarding working with PHP on a MAC?
Not at all. OS X is essentially Unix and you've already got Apache built in. I believe it comes with PHP installed (but not enabled) but I would recommend using Marc's installer from www.entropy.ch. This will give you the most current version of PHP (normally). In short, PHP is great on a Mac.
Hope that helps,
Larry