PHP无限极分类：如何用递归算法实现父子级分类的遍历与展示？

<?php
// 分类数据
$data = [
    [
        'id' => 1,
        'parent_id' => 0,
    ],
    [
        'id' => 2,
        'parent_id' => 1,
    ],
    [
        'id' => 3,
        'parent_id' => 1,
    ],
    [
        'id' => 4,
        'parent_id' => 2,
    ],
];

// 递归函数
function getCategories($data, $parent_id)
{
    $categories = [];
    foreach ($data as $item) {
        if ($item['parent_id'] == $parent_id) {
            $categories[] = $item;
            $categories = array_merge($categories, getCategories($data, $item['id']));
        }
    }
    return $categories;
}

// 获取顶级分类
$topCategories = getCategories($data, 0);

// 输出分类结构
foreach ($topCategories as $topCategory) {
    echo $topCategory['id'] . ': ' . $topCategory['parent_id'] . '<br>';
    foreach (getCategories($data, $topCategory['id']) as $childCategory) {
        echo '    ' . $childCategory['id'] . ': ' . $childCategory['parent_id'] . '<br>';
    }
}
?>