Bubble Sort Algorithm Explained with PHP Implementation

Bubble Sort, known as "Bubble Sort" in English and "泡沫排序" in Taiwan, is an in‑place, stable sorting algorithm with time complexity O(n²) and space complexity O(1).

When sorting in ascending order, the algorithm repeatedly compares adjacent elements and swaps them if they are out of order, moving the largest unsorted element to the end of the list after each pass.

The process continues until a full pass makes no swaps, indicating that the list is sorted.

Below is a PHP implementation of Bubble Sort that supports both ascending and descending order:

<?php
function bubble_sort($arr, $sort = true){
    if (empty($arr) || !is_array($arr)) {
        return false;
    }
    $count = count($arr);
    for ($i = 0; $i < $count; $i++) {
        for ($j = 1; $j < $count - $i; $j++) {
            if (($sort && $arr[$j - 1] > $arr[$j]) || (!$sort && $arr[$j - 1] < $arr[$j])) {
                list($arr[$j], $arr[$j - 1]) = array($arr[$j - 1], $arr[$j]);
            }
        }
    }
    return $arr;
}
$arr = [1, 6, 8, 10, 2, 5, 7, 9, 1];
echo "Original data:
";
print_r($arr);
echo "Sorted in ascending order:
";
print_r(bubble_sort($arr));
?>

The execution result shows the original array and the sorted array in ascending order:

Original data:
Array
(
    [0] => 1
    [1] => 6
    [2] => 8
    [3] => 10
    [4] => 2
    [5] => 5
    [6] => 7
    [7] => 9
    [8] => 1
)
Sorted in ascending order:
Array
(
    [0] => 1
    [1] => 1
    [2] => 2
    [3] => 5
    [4] => 6
    [5] => 7
    [6] => 8
    [7] => 9
    [8] => 10
)

Illustrative images from Wikipedia are included to help visualize the sorting process.

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