private static function _parse_experiments()
 {
     if (empty(self::$_experiments) && defined('UPFRONT_EXPERIMENTS_ON') && UPFRONT_EXPERIMENTS_ON) {
         $level = UPFRONT_EXPERIMENTS_ON;
         if (in_array($level, array(1, '1', true), true)) {
             self::$_experiments = self::LEVEL_DEFAULT;
         } else {
             self::$_experiments = $level;
         }
     }
 }