When you submit the form, the value is presented with debug_dump(), so strings are quoted and array values listed.

:
: - Stop the submitted form from bring processed
: - Don't process the form, redirect to another page (e.g. login after session timeout), and remember the values for when they return.

or back to examples 

<?php
	$field = new form_field_url($form, 'URL');
	$field->scheme_default_set('http');
	$field->scheme_allowed_set('Your url has an invalid scheme.', array('http', 'https'));
	$field->format_error_set('Your url does not appear to be correct.');
	$field->min_length_set('Your url is required.');
	$field->max_length_set('Your url cannot be longer than XXX characters.', 200);
?>