Manual talk:$wgDBprefix

I've been trying to use $wgDBPrefix in an extension I'm writing. First extension, newbie. Anyway, I added a new table to the database and added a couple of functions to manipulate it... they shouldn't affect anything else.

Problem: In the functions I attempt to use $wgDBPrefix to build the table names so that the code won't be hard-wired to my database. In one function, $wgDBPrefix returns (holds) the 3-letter prefix as expected. In another function, it returns not the prefix, but the name of the database. Can't figure out how or why this is getting munged... I'm not touching it. Thoughts?
 * Don't use that variable directly, there's DatabaseBase::tableName for that, along with query abstraction functions that do everything automatically: select, insert, update, delete and so on. Max Semenik 19:06, 13 November 2009 (UTC)

How do I add the code to the $wfDBprefix database. In other words how do I identifie what is the database location??