Here's an article that might help:
It could help you resolve the problem. But if you forward it to your client, it may help them appreciate that the time you will spend figuring out how to make it work could cover the cost of the first two years of hosting on a stable, compatible system. And you could be spending that time building a website, not debugging PHP on AIX.
If this client insists on using that system, I would run the other way. If that's how the project starts, it won't bode well.