How to...Free trial...

I would also extend the idea of the expire date to go beyond trials. If someone has paid for a year of service, the expire date would be set to one year after their payment (or one year beyond the existing expire date, if set and that behavior is preferred).

Additionally, a given user might want multiple domains, so the date should be tied to the domain, rather than the user.

As for the database setup for one database vs. multiple databases, I really just think that data for the domains setup for users should be separate from your site's user/domain information. A lot of this depends on what you're actually allowing them to do in the database, though.

Bonfire

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