Wordpress - WordPress mysqli and PDO

An update for Wordpress 3.9+ - with PHP 5.5 deprecating the mysql_* functions Wordpress has begun using the mysqli if it is available on the server. Your future plugins and code should take this into consideration and stop using mysql_*. Right now, in 3.9, wordpress checks for the availability of mysqli and uses it if available. If not it uses mysql_* but that check will be removed at the earliest possible convenience. Down the road it seems they will be using PDO but that is a larger project.

Here is an FAQ and announcement on it:

http://make.wordpress.org/core/2014/04/07/mysql-in-wordpress-3-9/

WordPress uses mysql_* functions.

http://core.trac.wordpress.org/browser/tags/3.5.1/wp-includes/wp-db.php

It has used those ever since I can remember, which probably answers your last question. It is what I would consider legacy code. I suspect it will be updated sometime soon (those functions were only deprecated as of PHP 5.5, I believe). I don't know which route development will go.

Link to a ticket in Trac, courtesy of Wyck (from a comment below):

http://core.trac.wordpress.org/ticket/21663