Cake\Network\Response::vary PHP Method

    public function vary($cacheVariances = null)
    {
        if ($cacheVariances !== null) {
            $cacheVariances = (array) $cacheVariances;
            $this->_headers['Vary'] = implode(', ', $cacheVariances);
        }
        if (isset($this->_headers['Vary'])) {
            return explode(', ', $this->_headers['Vary']);
        }
        return null;
    }