code: protected function applyStepping( $range, $step )
      {
          if ( $step === false || $step === 1 )
          {
              return $range;
          }
  
          foreach ( $range as $value => $tmp )
          {
              if ( ( $value % $step ) !== 0 )
              {
                  unset( $range[$value] );
              }
          }
          return array_values( $range );
      }
docs: Take a range array and apply a defined stepping to it.
  
  @param array $range Range array to apply the stepping to
  @param int $step Stepping to be applied
  @return array Array with the given stepping applied