You do not need to load any additional extension in order to use these functions.
These functions are new as of PHP 4.0.4 and might change their name in the near future. Suggestions are to change them to ctype_issomething() instead of ctype_somthing() or even to make them part of ext/standard and use their original C-names, although this would possibly lead to further confusion regarding the isset() vs. is_sometype() problem.