An array of callbacks, keyed by form field name, which make an assertion about a piece of
submitted form data. Each validator should accept the value of the form field submitted
(which will be modified by any applied filters), and return a boolean value indicating the
success of the validation. If a validator is specified with no key, it will receive all form
data and all database data. See the
_validate() method for more information.
protected $_validators = array();