Manual:$wgDBmysql4

Details
On versions of MediaWiki supporting MySQL 3.23.x, certain enhanced features like the boolean-mode fulltext search are used when this variable is set.

While still set for compatibility, the value is now ignored as of MediaWiki 1.6.0 since MySQL 3.x is no longer supported. From 1.6.0 the default value is set to true for compatibility reasons (so that extensions that check for the value of this setting get the right answer).