最长回文子串

expandAroundCenter($string, $i, $i);
            $len2 = $this->expandAroundCenter($string, $i, $i + 1);
            $max  = max($len1, $len2);
            if ($max > $end - $start) {
                $start = $i - intval(($max - 1) / 2);
                $end   = $i + intval($max / 2);
            }
        }
        $subStrLen = $end - $start + 1;

        return substr($string, $start, $subStrLen);
    }

    /**
     * @param $string
     * @param $l
     * @param $r
     *
     * @return int
     */
    private function expandAroundCenter($string, $l, $r)
    {
        while ($l >= 0 && $r < strlen($string) && $string[$l] == $string[$r]) {
            $l--;
            $r++;
        }

        return $r - $l - 1;
    }
}

$s   = new Solution();
$str = 'cbbd';
echo $s->longestPalindrome($str);