In the standard C locale letters are just [A-Za-z] and ctype_alpha() is equivalent to (ctype_upper($text) _BAR_ _BAR_ ctype_lower($text)), but other languages have letters that are considered neither upper nor lower case.
This extension is EXPERIMENTAL. The behaviour of this extension -- including the names of its functions and anything else documented about this extension -- may change without notice in a future release of PHP. Use this extension at your own risk.