You could also just try running your code on PHP4 and then fixing all the errors; generally that's easier for me to do since I know what works and what doesn't instead of fixing things that aren't broke ;)
If it ain't broke, don't fix it!
If it ain't broke, don't fix it.
In my experience, it sometimes helps to fix things that aren't broken; often you think of better ways to do things when you do.
That's not called fixing, that's called improving.
I suppose I should have worded that differently - what I meant was:
If you go back over your code and fix all the weird things you did when you didn't know any better, you might thing of something that surpasses the original point of the code.
Code revision, then :)