PHP4.04 adds a new dedicated character function Ctype_PHP tutorial

Character type functions
These functions check whether a character or string falls into a certain character class according to the i current locale.
These functions check whether a character or string falls into a certain character class according to the i current locale. Inside the class!

When called with an integer argument these functions behave exactly like their C counterparts.
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. Characters, returns true if each character in the string is equivalent to the request standard.

Passing anything else but a string or integer will return false immediately.

Passing anything else but a string or integer will return false immediately.

Warning
Warning:

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.

These functions are newly added in php4.04. May change in the near future. Suggestions include changing ctype_issomething() instead of ctype_somthing(), or even making them part of an extension/standard, and using their original C_names. Although this may lead to future confusion with isset() vs is_sometype().