querying more than one db from a php page?

Hi, I am developing my main site to run on php/mysql (currently lasso/filemaker). I am also developing some individual sites to be hosted on the same server as my main one and I'd like to be able to update those smaller site databases when activity happens on the main site. I'm still a couple of months away from tackling that part but can anyone predict trouble ahead, and if so, any remedies? MTIA George in Oxford/Edinburgh