Airship\Engine\Security\Util::charWhitelist PHP Method

    public static function charWhitelist(string $input, string $whitelist = self::PRINTABLE_ASCII) : string
    {
        $output = '';
        $length = self::stringLength($input);
        for ($i = 0; $i < $length; ++$i) {
            if (\strpos($whitelist, $input[$i]) !== false) {
                $output .= $input[$i];
            }
        }
        return $output;
    }