phpVirtualBox is not ready for 4.2 yet. Some things may work if they are backward compatible with 4.1. Do not create an issue report stating that phpvirtualbox is not compatible with a release candidate version of virtualbox. When you see phpvirtualbox-4.2-x, THAT version will be compatible with virtualbox 4.2.
I will try to get this done in the next few weeks. With my day job and family, I can't commit to any specific date.