Not sure what you're after.... maybe I'm missing your point.... but ALL form element values passed via POST or GET are ALWAYS passed as Strings. You can convert the passed values to integer, date, or whatever as needed, once the variables are requested on the page.... but they all get passed as strings.
I assume that what you are trying to do is make sure all characters entered are a-z or A-Z. I don't know of a specific function that will do this for you, but you can write one easily enough. Basically you "walk" through the string and check the ascii code for each letter and check to see if it falls into a certain range.