code: public static function UTF8toBIFF8UnicodeLong($value)
      {
          // character count
          $ln = self::countCharacters($value, 'UTF-8');
  
          // characters
          $chars = self::convertEncoding($value, 'UTF-16LE', 'UTF-8');
  
          $data = pack('vC', $ln, 0x0001) . $chars;
  
          return $data;
      }
docs: Converts a UTF-8 string into BIFF8 Unicode string data (16-bit string length)
  Writes the string using uncompressed notation, no rich text, no Asian phonetics
  If mbstring extension is not available, ASCII is assumed, and compressed notation is used
  although this will give wrong results for non-ASCII strings
  see OpenOffice.org's Documentation of the Microsoft Excel File Format, sect. 2.5.3.
  
  @param string $value UTF-8 encoded string
  
  @return string