code: public static function start($apiKey, $notifyOnWarning = false, array $options = array())
      {
          if (!isset(self::$instance)) {
              $config = new Configuration($apiKey, $options);
  
              $client = new Client($config);
              self::$instance = new self($client, $notifyOnWarning);
  
              if (null !== $config->get('errorReportingLevel')) {
                  self::$instance->addErrorFilter(new ErrorReporting($config));
              }
  
              self::$instance->addExceptionFilter(new AirbrakeExceptionFilter());
  
              set_error_handler(array(self::$instance, 'onError'));
              set_exception_handler(array(self::$instance, 'onException'));
              register_shutdown_function(array(self::$instance, 'onShutdown'));
          }
  
          return self::$instance;
      }
docs: Get the current handler.
  
  @param string $apiKey
  @param bool $notifyOnWarning
  @param array $options
  @return EventHandler