Анатолий Смилик

$arr = [1, 1, 2, 4, 3, 1, 1, 0, 1, 5, 8, 9, 1, 1, 2];
$direction = null;
$last = null;
$countMonotony = array_reduce(
    $arr,
    function ($total, $item) use (&$direction, &$last) {
        if (is_numeric($last)) {
            if ($item > $last && $direction != 'up') {
                $total++;
                $direction = 'up';
            } elseif ($item < $last && $direction != 'down') {
                $total++;
                $direction = 'down';
            } elseif ($item == $last) {
                $direction = null;
            }
        }

        $last = $item;

        return $total;
    }
);

var_dump($countMonotony); // int(6)