//<code>
// Prepare the empty array
$items = array();
// Prepare the database connection
$db = JFactory::getDbo();
// Keep this if you'd like a "Please select" option, otherwise comment or remove it
$items[] = "|Please Select[c]";
 
// Run the SQL query and store it in $results
$db->setQuery("SELECT title, title FROM jos0x_content WHERE catid = 13 or catid = 14");
$results = $db->loadObjectList();
 
// Now, we need to convert the results into a readable RSForm! Pro format.
// The Items field will accept values in this format:
// value-to-be-stored|value-to-be-shown
// Eg. m|M-sized T-shirt
foreach ($results as $result) {
  $value = $result->title;
  $label = $result->title;
  $items[] = $value.'|'.$label;
}
 
// Multiple values are separated by new lines, so we need to do this now
$items = implode("\n", $items);
 
// Now we need to return the value to the field
return $items;
//</code>