Create a global array..say $domain.
Put all domain info like domain Id nd name in that.
Now create a table with domain Id and related details.
At the application level you can identify every time with the global domain array.
Drupal has a module called domain which does the same functionality i guess.
Now you can also have option wat tables you want at domain level and wat u want to keep same.
do see and give it a try.
regards..
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