/**
* Add a unique key to the key index
*
* @internal
* @access private
* @param string key to add
*/
public static function add_key($key)
{
if (!is_array(self::$_keys)) {
self::$_keys = array();
}
if (!in_array($key, self::$_keys)) {
self::$_keys[] = $key;
}
}
