LJ::get_username — Returns a username given a userid.
dbarg. Deprecated; a master database handle ($dbh) or a master/slave set of database handles ($dbs).
user. Username whose userid to look up.
Results cached in memory. On miss, does DB call. Not advised to use this many times in a row... only once or twice perhaps per request. Tons of serialized db requests, even when small, are slow. Opposite of LJ::get_userid.