There are two web sites, lets call them a.com and b.com; they've been completely separate up until now. However, there are "synergies", so the client wants to make b.com part of a.com (same media library, users, host, database et cetera)... but with it's own domain and design. A site within the site, with its own domain.
SO, basically: when a user goes to a specific part of the site - let's say a.com/b, the URL should change to b.com.
I would think I can solve this with apache virtual hosts; I found this aswer for how to it with Yii: Assign different domain for two controllers only
But I'm using Wordpress (already made the main site), so I should go with vanilla PHP and Apache tweaking. Any tips and pointers are welcome.
Please note: This is not about multisite, as that would void the benefits of a common media gallery and users etc.