private function createMonthlyKey($prefix, \DateTime $datetime = null)
    {
        return sprintf("%s_%s", $prefix, $this->formatDate($datetime, 'Y_m'));
    }
private function createMonthlyKey($prefix, \DateTime $datetime = null)
    {
        return sprintf("%s_%s", $prefix, $this->formatDate($datetime, 'Y_m'));
    }
private function createMonthlyKey ( $ prefix , \ DateTime $ datetime = null ) { return sprintf ( "%s_%s" , $ prefix , $ this -> formatDate ( $ datetime , 'Y_m' ) ) ; }
Create the monthly key with prefix eg. 'total_2003_11'

@param string    $prefix
@param \DateTime $datetime

@return string
Create the monthly key with prefix eg . total_2003_11