These functions check whether a character or string falls into a certain character class according to the current locale.
When called with an integer argument these functions behave exactly like their C counterparts.
When called with a string argument they will check every character in the string and will only return true if every character in the string matches the requested criteria.
Passing anything else but a string or integer will return false immediately.
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_something() 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.