My host has informed us ahead of time that they are upgrading to PHP 5.3. I am running UBB Threads Version 7.5.4.2p2. Are there any compatibility problems or issues I need to be aware of?

Additionally, they also tell me that PHP 5.3 does not support Zend optimizer. Do we use that?

I run php 5.3 and yes it does not support zend optimizer but the replacement is zend gaurd loaded. Only issue i ran into with it was with gallery 3 software not ubb.

I doubt you use Zend optimizer if you don't know what it is... The UBB doesn't use it.

Well, the only issues I have come across have been some random php depreciation warnings in some other scripts. Just once in UBB... wish I thought to make a note of the warning and what I was doing.