public static function isSymbol($str)
    {
        if (strlen($str) === 0) {
            return null;
        }
        if ($str[0] === '@') {
            return Token::FLAG_SYMBOL_VARIABLE;
        } elseif ($str[0] === '`') {
            return Token::FLAG_SYMBOL_BACKTICK;
        } elseif ($str[0] === ':') {
            return Token::FLAG_SYMBOL_PARAMETER;
        }

        return null;
    }