PHP Form Validation: Validate values of several types

This class can validate values of several types.

It provides several functions to validate a given value according to a validation rule. The functions return a given error string if the rule is not satisfied.

Currently it provides validation rule functions to verify if the value contains letters with a length between a given minimum and a maximum, is a valid e-mail address, contains only digits, is a user name with alphanumeric characters with a minimum length, is a valid date, is a valid address, or is a valid password.