Now this really seems to be a complex and tricky issue with mySQL / database programming and configuration in general.
I never used to care a lot as long as I had to deal with smaller web applications on default shared hosting servers. Usually, anything left open would time or be automatically closed, and there was some "too many connections" very rarely, but also for out-of-the box software like bulletin boards and content management systems.

After installing XAMPP (lampp) on a local linux server and working with mySQL settings myself to optimize the system for more complex queries, I had to figure out the best way to set mysql query cache size, allowed number of maximum connections etc. Simply setting everything to very high values is definitely not the smartes way to do this. You have to think, watch your database design and queries, keep testing and watch the SQL status variables to find settings that match the project's requirements and hardware possibilities in the best possible way.