Calculate maximum nesting depth of parentheses

max-depth-parens.php

<?php

/**
 * Calculate maximum nesting depth of parentheses.
 *
 * @see https://www.youtube.com/watch?v=zrOIQEN3Wkk
 * @see https://leetcode.com/problems/maximum-nesting-depth-of-the-parentheses/
 */

$input = '(1+(2*3)+((8)/4))+1';

$depth = array_reduce(
    array_filter(str_split($input), fn($s) => strstr('()', $s)),
    function($carry, $s) {
        static $max = 0;
        $s === '(' ? $max++ : $max--;
        return $max > $carry ? $max : $carry;
    });

print_r($depth);