//<code>
JLoader::register('Services_JSON', JPATH_ADMINISTRATOR.DS.'components'.DS.'com_k2'.DS.'lib'.DS.'JSON.php');
$jinput = JFactory::getApplication()->input;

$nl = "\n";
$output = "|-- Select a product --".$nl;

$db = JFactory::getDBO();
$query = $db->getQuery(true);

$query = "SELECT p.id,p.title, p.extra_fields, p.alias, c.name AS category FROM `#__k2_items` AS p INNER JOIN (`#__k2_categories` AS c) ON (p.catid = c.id) WHERE p.published = 1 AND p.trash = 0  AND c.parent = 1 ORDER BY c.name ASC, TRIM(p.title) ASC";

$db->setQuery($query);
$products_list = $db->loadObjectList();

$p_cat = "";
$tot_prod = count($products_list);
$x = 1;

$json = new Services_JSON;
foreach($products_list AS $p){

	$jsonObjects = $json->decode($p->extra_fields);

	
        // $code is the the name of the only extra_fields that i need, but even the other extra_fields are inside the $jsonObject
        // so if you need another extra_field you can write: list($xf1,$xf2,$xf3,etc...,etc...,etc...) = $jsonObjects;
	list($code) = $jsonObjects; 
	
	if($p->category != $p_cat) {
		$output .= $p->category."[g]".$nl;
	}
	
$current_jinput_id = $p->id.$p->alias;
$checked = $jinput->get('id') == $current_jinput_id ? "[c]" : "";

	$output .= $p->id."|".$code->value." - ".$p->title."".$checked;

	if($x != $tot_prod) {
		$output .= $nl;
	}
	
	$p_cat = $p->category;
	$x++;
}
return $output;
//</code>