In most cases, there are not compatibility problems. However, you might want to read this. I don't mind, but some do - remember to google your problems before asking ;).
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 ;)
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.
APLX is a very complete implementation of the APL programming language from MicroAPL. The company stopped producing it in 2016 and it has been taken over by Dyalog. While Dyalog ...