code: public function isActive($scenario)
      {
          return !in_array($scenario, $this->except, true) && (empty($this->on) || in_array($scenario, $this->on, true));
      }
docs: Returns a value indicating whether the validator is active for the given scenario and attribute.
  
  A validator is active if
  
  - the validator's `on` property is empty, or
  - the validator's `on` property contains the specified scenario
  
  @param string $scenario scenario name
  @return bool whether the validator applies to the specified scenario.