Validator Class: Validate values reverting to defaults when invalid

This class is meant to validate page request values eventually submitted using forms via POST or GET HTTP methods.

The class provides functions that retrieve values submitted in a request variable of a given name and then performs the specified type of validation.

The class provides many built-in types of validation like: is it a valid number, integer, boolean, name of variable, it contains only letters, is it a valid e-mail adress, is it an integer within a given range, or even determine if it is a valid URL by trying to access it.

The class also lets the developers choose its behavior when the value being validated is invalid. The validation functions may return either an empty string, null, zero, or any other default value.