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

* Items:
: - 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
	$items = array(
		'A' => 'Item A',
		'B' => 'Item B',
		'C' => 'Item C',
	);
	$field = new form_field_radios($form, 'Items');
	$field->options_set($items);
	$field->required_error_set('An item is required.');
	// $value = $this->value_get();
	// $value = $this->value_key_get();
?>