Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/StringHelper.php
public static function controlCharacterPHP2OOXML($value)
    {
        self::buildCharacterSets();

        return str_replace(array_values(self::$controlCharacters), array_keys(self::$controlCharacters), $value);
    }
public static function controlCharacterPHP2OOXML($value)
    {
        self::buildCharacterSets();

        return str_replace(array_values(self::$controlCharacters), array_keys(self::$controlCharacters), $value);
    }
Convert from PHP control character to OpenXML escaped 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 escape

@return string