Manual:$wgDBmysql5

Details
Set to true to engage MySQL 4.1/5.0 charset-related features; for now will just cause sending of 'SET NAMES=utf8' on connect.

WARNING: THIS IS EXPERIMENTAL!

May break if you're not using the table defs from mysql5/tables.sql. May break if you're upgrading an existing wiki if set differently. Broken symptoms likely to include incorrect behavior with page titles, usernames, comments etc containing non-ASCII characters. Might also cause failures on the object cache and other things.

Even correct usage may cause failures with Unicode supplementary characters (those not in the Basic Multilingual Plane) unless MySQL has enhanced their Unicode support.