code: public static function controlCharacterOOXML2PHP($value)
      {
          self::buildCharacterSets();
  
          return str_replace(array_keys(self::$controlCharacters), array_values(self::$controlCharacters), $value);
      }
docs: Convert from OpenXML escaped control character to PHP control character.
  
  Excel 2007 team:
  ----------------
  That's correct, control characters are stored directly in the shared-strings table.
  We do encode characters that cannot be represented in XML using the following escape sequence:
  _xHHHH_ where H represents a hexadecimal character in the character's value...
  So you could end up with something like _x0008_ in a string (either in a cell value (<v>)
  element or in the shared string <t> element.
  
  @param string $value Value to unescape
  
  @return string