code: public function writeBlank($row, $col, $xfIndex)
      {
          $record = 0x0201; // Record identifier
          $length = 0x0006; // Number of bytes to follow
  
          $header = pack('vv', $record, $length);
          $data = pack('vvv', $row, $col, $xfIndex);
          $this->append($header . $data);
  
          return 0;
      }
docs: Write a blank cell to the specified row and column (zero indexed).
  A blank cell is used to specify formatting without adding a string
  or a number.
  
  A blank cell without a format serves no purpose. Therefore, we don't write
  a BLANK record unless a format is specified.
  
  Returns  0 : normal termination (including no format)
  -1 : insufficient number of arguments
  -2 : row or column out of range
  
  @param int $row Zero indexed row
  @param int $col Zero indexed column
  @param mixed $xfIndex The XF format index
  
  @return int