code: public function assertInteger($config, $key, $lowest = null, $highest = null)
      {
          if (array_key_exists($key, $config)) {
              if (!is_int($config[$key])) {
                  throw new ConfigurationException($key." must be an integer.");
              }
              if ($lowest !== null && $config[$key] < $lowest) {
                  throw new ConfigurationException($key." cannot be lower than ".$lowest);
              }
              if ($highest !== null && $config[$key] > $highest) {
                  throw new ConfigurationException($key." cannot be higher than ".$highest);
              }
          }
          return $this;
      }
docs: Assert the property is integer or is not set
  @param array $config Array being validated
  @param string|int $key Inspected array key
  @param int $lowest The lowest accepted values
  @param int $highest The highest accepted value
  @return $this
  @throws ConfigurationException On a failed assertation