boundary.php

<?php

    public static function isUpperBoundary(array $input, int $current_key, $pattern, int $steps_back): bool
    {
        for ($counter = $current_key - 1; $counter >= $current_key - $steps_back; $counter--) {
            if (!array_key_exists($counter, $input) || $input[$counter] !== $pattern) {
                return false;
            }
        }
        return true;
    }

    public static function isLowerBoundary(array $input, int $current_key, $pattern, int $steps_forward): bool
    {
        for ($counter = $current_key + 1; $counter <= $current_key + $steps_forward; $counter++) {
            if (!array_key_exists($counter, $input) || $input[$counter] !== $pattern) {
                return false;
            }
        }
        return true;
    }