Thanks for that info.
I left out an important part of my question:
While each of the websites uses the same geographic data (zip codes, city/state geocodes, etc.) each website has it's own version of data specific to that website. For instance, one site maintains a list of grow stores and has a table 'locations' that contains all of those locations. Another site maintains a list of motorsports shops with it's own, structurally different version of the table 'locations'.
If each site has it's own db and also pulls from a shared db, two MySQL db connections would often be required instead of one.
Is the processing cost of 2 PDO/MySQL connections substantial enough to warrant keeping multiple copies of the same data in multiple databases? Or is the cost of 2 db connections so insignificant that it makes sense for 4 or 5 sites to use a shared database AND have their own unique database?
My concerns are 1. Additional processing required to use 2 database connections per site and 2. Possible delays due to multiple sites sharing the same database tables vs. each site using it's own copy.
Thanks for your input!