Variable (Class instantiation) collision

I am running into a variable collision. The project I'm developing is NOT guaranteed to be operating on PHP5. Any solution I find should (hopefully) be able to run on PHP4 (yes, I know PHP4 is deprecated). I am building a bridge between two third-party applications. Both instantiate their respective database class assigning it to $db and I cannot change that. So, I am interested in solutions to this. I found a reference to a Packager class and will be looking at it shortly.