Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/TextData.php
public static function RETURNSTRING($testValue = '')
    {
        $testValue = Functions::flattenSingleValue($testValue);

        if (is_string($testValue)) {
            return $testValue;
        }

        return null;
    }
public static function RETURNSTRING($testValue = '')
    {
        $testValue = Functions::flattenSingleValue($testValue);

        if (is_string($testValue)) {
            return $testValue;
        }

        return null;
    }
RETURNSTRING.

@param mixed $testValue Value to check

@return null|string