The default transformation will be uppercase as in mb_convert_case [1].
Possible modes are:
lower
Transforms the input string to its lowercase representation
upper
Transforms the input string to its uppercase representation
capital
Transforms the input string to its first letter upper-cased, i.e. capitalization
uncapital
Transforms the input string to its first letter lower-cased, i.e. uncapitalization
capitalWords
Transforms the input string to each containing word being capitalized
Note that the behavior will be the same as in the appropriate PHP function mb_convert_case [1];
especially regarding locale and multibyte behavior.