code: public static function absoluteReference($pCoordinateString)
      {
          if (self::coordinateIsRange($pCoordinateString)) {
              throw new Exception('Cell coordinate string can not be a range of cells');
          }
  
          // Split out any worksheet name from the reference
          $worksheet = '';
          $cellAddress = explode('!', $pCoordinateString);
          if (count($cellAddress) > 1) {
              list($worksheet, $pCoordinateString) = $cellAddress;
          }
          if ($worksheet > '') {
              $worksheet .= '!';
          }
  
          // Create absolute coordinate
          if (ctype_digit($pCoordinateString)) {
              return $worksheet . '$' . $pCoordinateString;
          } elseif (ctype_alpha($pCoordinateString)) {
              return $worksheet . '$' . strtoupper($pCoordinateString);
          }
  
          return $worksheet . self::absoluteCoordinate($pCoordinateString);
      }
docs: Make string row, column or cell coordinate absolute.
  
  @param string $pCoordinateString e.g. 'A' or '1' or 'A1'
  Note that this value can be a row or column reference as well as a cell reference
  
  @throws Exception
  
  @return string Absolute coordinate        e.g. '$A' or '$1' or '$A$1'