PHP4.04 added the special character function Ctype.

Depends. these functions check whether a character or string is in a character class based on the current field I! 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 is in a character class based on the current field I!

When called with an integer argument these funese behave exactly like their C counterparts.
When an integer parameter is called, these functions exactly match the functions in C.

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.
When the string parameter is used, each character in the string is checked. if each character in the string is equivalent to the request standard, true is returned.

Passing anything else but a string or integer will return false immediately.
If you send a parameter that is not a string or integer, false is returned 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 wowould possibly lead to further confusion regarding the isset (). is_sometype () problem.

These functions are newly added to php4.04. It may change recently. We recommend that you change ctype_issomething () to replace ctype_somthing (), even make them part of the extension/standard, and use their original C _ name. Although it may cause confusion between isset () and is_sometype () in the future.