Simplify substr usages

Author: Jean85

src/PhpSpreadsheet/Writer/Xls/BIFFwriter.php

@@ -217,7 +217,7 @@ private function addContinue($data)
         // Retrieve the last chunk of data
         $header = pack('vv', $record, strlen($data) - $i);
         $tmp .= $header;
-        $tmp .= substr($data, $i, strlen($data) - $i);
+        $tmp .= substr($data, $i);
 
         return $tmp;
     }

src/PhpSpreadsheet/Writer/Xls/Parser.php

@@ -561,7 +561,7 @@ private function convertNumber($num)
     private function convertString($string)
     {
         // chop away beggining and ending quotes
-        $string = substr($string, 1, strlen($string) - 2);
+        $string = substr($string, 1, -2);
         if (strlen($string) > 255) {
             throw new WriterException('String is too long');
         }