I just pushed ProcessWire v2.3.1 to the dev branch. This is a fairly major change in that it switches the DB driver from mysqli to PDO. It meant changes to a large percentage of core files. ProcessWire still supports mysqli, but doesn't attempt to use it unless a module or a template asks for it via the $db API variable. The new API variable (for the PDO driver) is $database. More about PDO at php.net If you are using the dev branch, be careful and test thoroughly with this latest commit to it
