valleys.php

<?php

/**
 * Usage : php valleys.php 8 DDUUUUDD
 *
 * 1
 */

function countValleys(string $path): int
{
    $res = array_reduce(str_split($path), static function (array $carry, string $step) {
        if (!in_array($step, ['U', 'D'], true)) {
            throw new \InvalidArgumentException('The step must be U or D');
        }
        $carry['altitude'] = ($carry['altitude'] ?? 0) + ('U' === $step ? 1 : -1);
        $carry['valleys'] = ($carry['valleys'] ?? 0) + ('U' === $step && 0 === $carry['altitude'] ? 1 : 0);

        return $carry;
    }, []);

    return $res['valleys'] ?? 0;
}

$steps = (int)($argv[1] ?? 0);
$path = (string)($argv[2] ?? '');

if (!$steps || $steps !== mb_strlen($path)) {
    throw new \InvalidArgumentException('You must provide a valid step & path values');
}

echo countValleys($path);