public static function convertToBinary($string, $characters) {
        if (empty($string) || empty($characters)) {
            return '';
        }
        $string   = str_split($string);
        $callback = function($str) use ($characters) {
                return strpos($characters, $str);
            };
        $string    = array_map($callback, $string);
        $converted = static::baseConvert($string, strlen($characters), 256);
        $callback  = function ($num) {
                return chr($num);
            };
        return implode('', array_map($callback, $converted));
    }