public function setLevels($levels)
{
if (is_integer($levels)) {
$this->_levels = $levels;
} else {
$this->_levels = null;
$levels = strtolower($levels);
foreach (explode(',', $levels) as $level) {
$level = trim($level);
if (isset(self::$_levelValues[$level])) {
$this->_levels |= self::$_levelValues[$level];
}
}
}
}