Почему мой заголовок Content-Length неверен?

Я пытаюсь узнать точную длину строки, используя strlen() в php 5.2. Строка ($data) содержит '\t' и '\n'.

echo strlen($data);

Код:

    // fetch table header
      $header = '';
      while ($fieldData = $result->fetch_field()) {
        $header .= $fieldData->name . "\t";
      }

      // fetch data each row, store on tabular row data
      while ($row = $result->fetch_assoc()) {
        $line = '';
        foreach($row as $value){
          if(!isset($value) || $value == ""){
            $value = "\t";
          }else{
            // important to escape any quotes to preserve them in the data.
            $value = str_replace('"', '""', $value);
            // needed to encapsulate data in quotes because some data might be multi line.
            // the good news is that numbers remain numbers in Excel even though quoted.
            $value = '"' . $value . '"' . "\t";
          }

          $line .= $value;
        }
        $data .= trim($line)."\n";
      }

      // this line is needed because returns embedded in the data have "\r"
      // and this looks like a "box character" in Excel
      $data = str_replace("\r", "", $data);

      // Nice to let someone know that the search came up empty.
      // Otherwise only the column name headers will be output to Excel.
      if ($data == "") {
        $data = "\nno matching records found\n";
      }

      // create table header showing to download a xls (excel) file
      header("Content-type: application/octet-stream");
      header("Content-Disposition: attachment; filename=$export_filename");
      header("Cache-Control: public");
      header("Content-length: " . strlen($data); // tells file size
      header("Pragma: no-cache");
      header("Expires: 0");

  // output data
  echo $header."\n".$data;

Это не возвращает точную длину (она меньше фактической длины). Пожалуйста посоветуй.