Does this mean my OS is equivalent to Ubuntu 12?
Zorin OS 12.3 64-bit
gnome-shell --version
shell 3.18.5
If I download the latest Lite what will I have?
With zorin 15.3 you would have the equivalent of xubuntu 18.04 with some of the updates from 20.03 i think.
Zorin OS 12, is based off of Ubuntu 16.04. It appears that your OS hasn't been updated for awhile, your running OS 12.3, but 12.4 has been around for 2-years I believe.
Now, as far as doing a complete OS upgrade to Zorin OS 15.3, Harvey is correct, its based off of Ubuntu 18.04.
However, I would recommend that you please hold off on upgrading for now. Zorin OS 16 has been in production for awhile, and will soon have a full release. Better to wait, and install the latest version.
So go ahead and hold out for awhile longer.
Will it have php preinstalled?
Apologies, for I am not one of the developers, and cannot answer that question, based on the Alpha state of the OS.
What I can tell you is this, once Zorin OS 16 Ultimate is released, it will contain most of what everybody needs.