Note that although "multi-byte" hints at total internationalization, the mb_ API was designed by a Japanese person to support the Japanese language.
Some of the functions, for example mb_convert_kana(), make absolutely no sense outside of a Japanese language environment.
It should perhaps be considered "lucky" if the functions work with non-Japanese multi-byte languages.
I don't mean any disrespect to the mb_ API because I'm using it everyday and I appreciate its usefulness, but maybe a better name would be the jp_ API.
