Custom Form Validation: Validate form values according to several rules

This class can be used to validate form values according to several rules.

It can check field values of forms submitted using the POST method and perform several types of validation.

Invalid values are associated to given error messages.

Currently it can validate field values with rules: has alphanumeric characters, is a date or time, is an e-mail, has only numbers, has an exact or minimum length, is different from another field, is not empty, is a phone number, is an URL, or is an social security number.

It may also clean values to prevent cross-site scripting attacks.