以下是一个使用PHP实现的无限级联实例，通过递归函数来构建多级联动的下拉菜单。
```php
// 假设我们有一个分类表，包含分类ID和父分类ID
$categories = [
['id' => 1, 'parent_id' => 0, 'name' => '一级分类1'],
['id' => 2, 'parent_id' => 0, 'name' => '一级分类2'],
['id' => 3, 'parent_id' => 1, 'name' => '二级分类1-1'],
['id' => 4, 'parent_id' => 1, 'name' => '二级分类1-2'],
['id' => 5, 'parent_id' => 2, 'name' => '二级分类2-1'],
['id' => 6, 'parent_id' => 2, 'name' => '二级分类2-2'],
['id' => 7, 'parent_id' => 3, 'name' => '三级分类1-1-1'],
['id' => 8, 'parent_id' => 3, 'name' => '三级分类1-1-2'],
['id' => 9, 'parent_id' => 4, 'name' => '三级分类1-2-1'],
['id' => 10, 'parent_id' => 4, 'name' => '三级分类1-2-2'],
['id' => 11, 'parent_id' => 5, 'name' => '三级分类2-1-1'],
['id' => 12, 'parent_id' => 5, 'name' => '三级分类2-1-2'],
['id' => 13, 'parent_id' => 6, 'name' => '三级分类2-2-1'],
['id' => 14, 'parent_id' => 6, 'name' => '三级分类2-2-2'],
];
function buildCascadingDropdown($categories, $parentId = 0, $level = 0) {
$options = '
