code: public static function start(string $key = 'default'): void {
      if (isset(self::$timers[$key])) {
        self::$timers[$key]->start();
      }
      else {
        self::$timers[$key] = new Stopwatch();
      }
    }
docs: Start or resume the timer.
  
  Call this method to start the timer with a given key. The default key
  is "default", and used in @see \Ayesh\PHP_Timer\Timer::read() and reset()
  methods as well
  
  Calling this with the same $key will not restart the timer if it has already
  started.
  
  @param string $key