Solution.php

class Solution {
    /**
     * @param string $s
     * @return string
     */
    function longestPalindrome($s) {
        $start = 0;
        $maxLength = 0;

        for ($i = 0; $i < strlen($s); $i++) {
            $len1 = $this->expandFromCenter($s, $i, $i);
            $len2 = $this->expandFromCenter($s, $i, $i + 1);

            $len = max($len1, $len2);

            if ($len > $maxLength) {
                $start = $i - intval(($len - 1) / 2);
                $maxLength = $len;
            }
        }

        return substr($s, $start, $maxLength);
    }

    function expandFromCenter($s, $left, $right) {
        while ($left >= 0 && $right < strlen($s) && $s[$left] === $s[$right]) {
            $left--;
            $right++;
        }

        return $right - $left - 1;
    }
}