diff --git a/src/PhpSpreadsheet/Writer/Xls.php b/src/PhpSpreadsheet/Writer/Xls.php
index c17feedce..f9a0a3d8c 100644
--- a/src/PhpSpreadsheet/Writer/Xls.php
+++ b/src/PhpSpreadsheet/Writer/Xls.php
@@ -190,7 +190,7 @@ class Xls extends BaseWriter
         $worksheetSizes = [];
         for ($i = 0; $i < $countSheets; ++$i) {
             $this->writerWorksheets[$i]->close();
-            $worksheetSizes[] = $this->writerWorksheets[$i]->_datasize;
+            $worksheetSizes[] = $this->writerWorksheets[$i]->getBiffDataSize();
         }
 
         // add binary data for global workbook stream
diff --git a/src/PhpSpreadsheet/Writer/Xls/BIFFwriter.php b/src/PhpSpreadsheet/Writer/Xls/BIFFwriter.php
index a9097835c..ae8fa6bb9 100644
--- a/src/PhpSpreadsheet/Writer/Xls/BIFFwriter.php
+++ b/src/PhpSpreadsheet/Writer/Xls/BIFFwriter.php
@@ -19,12 +19,12 @@ class BIFFwriter
     /**
      * The string containing the data of the BIFF stream.
      */
-    public ?string $_data;
+    protected ?string $biffData;
 
     /**
-     * The size of the data in bytes. Should be the same as strlen($this->_data).
+     * The size of the data in bytes. Should be the same as strlen($this->biffData).
      */
-    public int $_datasize;
+    protected int $biffDataSize;
 
     /**
      * The maximum length for a BIFF record (excluding record header and length field). See addContinue().
@@ -38,8 +38,8 @@ class BIFFwriter
      */
     public function __construct()
     {
-        $this->_data = '';
-        $this->_datasize = 0;
+        $this->biffData = '';
+        $this->biffDataSize = 0;
     }
 
     /**
@@ -76,12 +76,12 @@ class BIFFwriter
         if (strlen($data) - 4 > $this->limit) {
             $data = $this->addContinue($data);
         }
-        $this->_data .= $data;
-        $this->_datasize += strlen($data);
+        $this->biffData .= $data;
+        $this->biffDataSize += strlen($data);
     }
 
     /**
-     * General storage function like append, but returns string instead of modifying $this->_data.
+     * General storage function like append, but returns string instead of modifying $this->biffData.
      *
      * @param string $data binary data to write
      */
@@ -90,7 +90,7 @@ class BIFFwriter
         if (strlen($data) - 4 > $this->limit) {
             $data = $this->addContinue($data);
         }
-        $this->_datasize += strlen($data);
+        $this->biffDataSize += strlen($data);
 
         return $data;
     }
@@ -181,4 +181,9 @@ class BIFFwriter
 
         return $tmp;
     }
+
+    public function getBiffDataSize(): int
+    {
+        return $this->biffDataSize;
+    }
 }
diff --git a/src/PhpSpreadsheet/Writer/Xls/Workbook.php b/src/PhpSpreadsheet/Writer/Xls/Workbook.php
index d159cab9c..6c4c6bce7 100644
--- a/src/PhpSpreadsheet/Writer/Xls/Workbook.php
+++ b/src/PhpSpreadsheet/Writer/Xls/Workbook.php
@@ -427,9 +427,9 @@ class Workbook extends BIFFwriter
         }
 
         // Add part 3 of the Workbook globals
-        $this->_data .= $part3;
+        $this->biffData .= $part3;
 
-        return $this->_data;
+        return $this->biffData;
     }
 
     /**
@@ -440,7 +440,7 @@ class Workbook extends BIFFwriter
         $boundsheet_length = 10; // fixed length for a BOUNDSHEET record
 
         // size of Workbook globals part 1 + 3
-        $offset = $this->_datasize;
+        $offset = $this->biffDataSize;
 
         // add size of Workbook globals part 2, the length of the SHEET records
         $total_worksheets = count($this->spreadsheet->getAllSheets());
diff --git a/src/PhpSpreadsheet/Writer/Xls/Worksheet.php b/src/PhpSpreadsheet/Writer/Xls/Worksheet.php
index 6da5a81ee..022d98721 100644
--- a/src/PhpSpreadsheet/Writer/Xls/Worksheet.php
+++ b/src/PhpSpreadsheet/Writer/Xls/Worksheet.php
@@ -605,9 +605,9 @@ class Worksheet extends BIFFwriter
     public function getData(): string
     {
         // Return data stored in memory
-        if (isset($this->_data)) {
-            $tmp = $this->_data;
-            $this->_data = null;
+        if (isset($this->biffData)) {
+            $tmp = $this->biffData;
+            $this->biffData = null;
 
             return $tmp;
         }