PHP4.04 新增加了专用的字符函数 Ctype

Character type functions
These functions check whether a character or string falls into a certain character class according to the i current locale.
这些函数根据当前现场的 i 检查一个字符或字符串是否在一个字符类里面!

When called with an integer argument these functions behave exactly like their C counterparts.
当使用整数参数调用时，这些函数和 C 里面的函数完全匹配。

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.
当使用字符串参数，将检查字符串里面的每一个字符，如果字符串里面的每个字符都等同于请求标准时返回 true.

Passing anything else but a string or integer will return false immediately.
传送非字符串或整数的参数将立即返回 false

Warning
警告：

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.

这些函数是 php4.04 新增加的。可能在近期改变。建议包括更改 ctype_issomething() 代替 ctype_somthing()，甚至让他们成为扩展/标准的一部分，和使用他们的原始 C_名字。尽管可能导致将来 isset() 对 is_sometype() 的混乱。