private function createYearlyKey($prefix, \DateTime $datetime = null)
    {
        return sprintf("%s_%s", $prefix, $this->formatDate($datetime, 'Y'));
    }
private function createYearlyKey($prefix, \DateTime $datetime = null)
    {
        return sprintf("%s_%s", $prefix, $this->formatDate($datetime, 'Y'));
    }
private function createYearlyKey ( $ prefix , \ DateTime $ datetime = null ) { return sprintf ( "%s_%s" , $ prefix , $ this -> formatDate ( $ datetime , 'Y' ) ) ; }
Create the yearly key with prefix eg. 'total_2003'

@param string    $prefix
@param \DateTime $datetime

@return string
Create the yearly key with prefix eg . total_2003