public function parseKeyword()
    {
        $token = '';

        /**
         * Value to be returned.
         *
         * @var Token
         */
        $ret = null;

        /**
         * The value of `$this->last` where `$token` ends in `$this->str`.
         *
         * @var int
         */
        $iEnd = $this->last;

        /**
         * Whether last parsed character is a whitespace.
         *
         * @var bool
         */
        $lastSpace = false;

        for ($j = 1; $j < Context::KEYWORD_MAX_LENGTH && $this->last < $this->len; ++$j, ++$this->last) {
            // Composed keywords shouldn't have more than one whitespace between
            // keywords.
            if (Context::isWhitespace($this->str[$this->last])) {
                if ($lastSpace) {
                    --$j; // The size of the keyword didn't increase.
                    continue;
                }
                $lastSpace = true;
            } else {
                $lastSpace = false;
            }

            $token .= $this->str[$this->last];
            if (($this->last + 1 === $this->len || Context::isSeparator($this->str[$this->last + 1]))
                && $flags = Context::isKeyword($token)
            ) {
                $ret = new Token($token, Token::TYPE_KEYWORD, $flags);
                $iEnd = $this->last;

                // We don't break so we find longest keyword.
                // For example, `OR` and `ORDER` have a common prefix `OR`.
                // If we stopped at `OR`, the parsing would be invalid.
            }
        }

        $this->last = $iEnd;

        return $ret;
    }