bubble_recursive.php

<?php

function bubble_recursive(array $items, $iterations = null) {
    if ($iterations === null) {
        $iterations = count($items);
    }

    $items = bubble_recursive_inner($items, $iterations);

    if ($iterations > 1) {
        return bubble_recursive($items, $iterations - 1);
    }

    return $items;
}

function bubble_recursive_inner(array $items, $iterations, $i = 1) {
    if ($i < $iterations) {
        $prev = $items[$i - 1];
        $next = $items[$i];

        if ($prev > $next) {
            $items[$i] = $prev;
            $items[$i - 1] = $next;
        }

        return bubble_recursive_inner($items, $iterations, $i + 1);
    }

    return $items;
}

$items = [1, 6, 2, 5, 3, 4];

bubble_recursive($items); // [1, 2, 3, 4, 5, 6]