Just a suggestion here; but if you’re gonna get into sharing identities (i.e. memberships) across a number of sites, am wondering if you’ve considered leveraging a technology like “Live ID” (used to be called Microsoft Passport). Clearly you’re a fan of PHP, but if you’re hosting on Windows (or willing to migrate content to a Windows platform), Live ID could offer a number of options that would make the type of transient memberships traditionally a source of inconvenience for patrons of adult entertainment sites a lot easier to manage.

There’s lots of material on the web concerning Live ID, of course - and it’s been heavily revamped by Microsoft since the days of Passport to use new industry standard authentication methods. I’ve used the technology myself, and could answer specific questions if you’d like - but thought I’d put forth the idea here since you cited interest in shared memberships.