MediaWiki:Config-mysql-charset-help/en

In binary mode, MediaWiki stores UTF-8 text to the database in binary fields. This is more efficient than MySQL's UTF-8 mode, and allows you to use the full range of Unicode characters.

In UTF-8 mode, MySQL will know what character set your data is in, and can present and convert it appropriately, but it will not let you store characters above the [//en.wikipedia.org/wiki/Mapping_of_Unicode_character_planes Basic Multilingual Plane].