Example form is linked to a database table, but won't be saved in this example.
When you submit the form, the value is presented with debug_dump(), so strings are quoted and array values listed.
<?php
$field = new form_field_email($form, 'Email');
$field->db_field_set('email');
// $field->domain_check_set(false);
// $field->domain_error_set('Your email does not appear to be for a valid domain.', 'Tick if you are sure this is valid.');
$field->format_error_set('Your email does not appear to be correct.');
$field->min_length_set('Your email is required.');
$field->max_length_set('Your email cannot be longer than XXX characters.');
?>