I wanted to get some input on a design... specifics aside, the idea is to be able to create numerous child sites under the main site.. now, an idea is obviously to have all child sites share the database together to beable to give access to all users on each site access to other child sites and move freely across the network without having to register and what not.. the benefits to this specific network would be great
Cons, obviously a very large database with large amounts of queries - security is a big concern though not really as I have that somewhat taken care of.. Cache common database queries, restricting access, not allowing child sites to upload php files, etc.
The real question is has anybody taken on a project that has a similar idea and run into problems or issues that most people wouldn't think of and how it was dealt with...
I know I would save a lot of time and effort by just giving each child site it's own database and letting them do whatever they want...
You don't really give much detail so this obviously depends on what the sites will be doing but I'd just have a database for each site - this way if the load on your main database server gets too high then you can simply move the heavy databases to another DB server.
If you were in fact implying using 1 central database for authentication of various sites then I'd probably look at OpenLDAP or something.
If you can post more specifics we can be of more help.