$form
$form :
Store the complete form data if validation is coming from a form.
Validators are functions used to validate user/program input.
A validator signature is: my_validator($field_data, $params=array()) with $params an associative array of parameters.
A validator must fail on an empty string by raising an Pluf_Form_Invalid Exception or return the data in the right format (string, bool, whatever).
FIXME: Escape the strings when bad strings are sent in the error message.