I prefer constants to be all caps, OO Classes to be mixed, and pretty much everything else to be all lower. Maybe more caps for global data, but generally speaking I avoid it.

Regardless of case, that is a horrible mismatch. SUN and HP refer to companies, and AIX refers to an OS from IBM. I guess Linux could go either way The hobgoblins in my mind would like some consistency here. I'd go with OS over company (ie Sun => Solaris, HP => HP-UX) but that is your call.

As for question #2, why not simply add_account($server) and have that call get_os_type()? Doesn't this solve the problem by only exposing a single prototype that works for all OS types?