Hi,
I just had the same problem when trying to upgrade to ZenPhoto 1.4.5.1.
My host has php 5.3.26 and mysql 5.5.30.
Now, although mysqli is installed and enabled, I couldn't get it to work with ZP (I don't remember if in the older versions I used it or mysql driver). The thing is, that during setup, I could select the mysql driver, and then that step would work, but the next setup step (for example, delete extra file) would revert back to using mysqli and thus fail to connect again.
This happened even if I edited the config file manually. It seems there's a bug in the setup script which overwrites the config at every step and reverts it back to using mysqli.
The workaround I found for this was to manually edit the config file to change it to use mysql driver, before continuing to the next step, [b]for each step[/b].
It would be nice if I found why mysqli doesn't work on my host with ZP, but unfortunately I don't have time to debug this. I had to get this upgrade done and move on with my life

But in any case, it is definite that there's a bug in the setup script which overwrites the user selection of DB driver, and this happens at every step of the setup.
Cheers,
o