public function evenKey(array $storage)
    {
        return $this->filter($storage, function ($value, $key)
        {
            return $this->helper->math()->isEven($key);
        });
    }
public function evenKey(array $storage)
    {
        return $this->filter($storage, function ($value, $key)
        {
            return $this->helper->math()->isEven($key);
        });
    }
public function evenKey ( array $ storage ) { return $ this -> filter ( $ storage , function ( $ value , $ key ) { return $ this -> helper -> math ( ) -> isEven ( $ key ) ; } ) ; }
1=>'', 3=>''...

@param array $storage

@return array
1 = > 3 = > ...