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
	$field = new form_field_fields($form, 'Estimate');
	$field->input_add('V', ['size' => 3]);
	$field->input_add('U', array('options' => ['hours' => 'Hours', 'days' => 'Days']));
	$field->required_error_set('The estimate is required.');
	$field->invalid_error_set('An invalid estimate value has been set.');
?>