public function parseSymbol()
    {
        $token = $this->str[$this->last];
        if (! ($flags = Context::isSymbol($token))) {
            return null;
        }

        if ($flags & Token::FLAG_SYMBOL_VARIABLE) {
            if ($this->last + 1 < $this->len && $this->str[++$this->last] === '@') {
                // This is a system variable (e.g. `@@hostname`).
                $token .= $this->str[$this->last++];
                $flags |= Token::FLAG_SYMBOL_SYSTEM;
            }
        } elseif ($flags & Token::FLAG_SYMBOL_PARAMETER) {
            if ($this->last + 1 < $this->len) {
                ++$this->last;
            }
        } else {
            $token = '';
        }

        $str = null;

        if ($this->last < $this->len) {
            if (($str = $this->parseString('`')) === null) {
                if (($str = $this->parseUnknown()) === null) {
                    $this->error(
                        'Variable name was expected.',
                        $this->str[$this->last],
                        $this->last
                    );
                }
            }
        }

        if ($str !== null) {
            $token .= $str->token;
        }

        return new Token($token, Token::TYPE_SYMBOL, $flags);
    }