Oh, no, I definetly don't want to config 2 sets of PHP on one server, it just make things complicated and may bring extra trouble later.
I like php5, the speed is faster than php 4 and has new functions i would chose to use.
I don't like mysql5, the speed is slow, and the new functions i most likely will to to use.
so i only have 2 choices: php4+mysql4( orginal setup), or upgrade to php5+mysql5.
tough call.
