Converting Arrays to Tree Structures and Back in PHP

This article from the PHP Chinese community presents a tutorial on converting an array to a tree structure and back using PHP.

First, an example controller method public function index() creates a sample data array with id, parent_id, and name fields, then calls $this->list_to_tree($data) and dumps the result.

public function index()
{
    $data = [
        ['id'=>1, 'parent_id'=>0, 'name'=>'第一个'],
        ['id'=>2, 'parent_id'=>0, 'name'=>'第二个'],
        ['id'=>3, 'parent_id'=>1, 'name'=>'第三个'],
    ];
    $r = $this->list_to_tree($data);
    dump($r);
}

The list_to_tree function builds a hierarchical tree by first creating a reference map of items keyed by their primary key, then iterating over the list to attach each item to its parent based on parent_id. Items whose parent matches the root are added to the top‑level $tree array; otherwise they are placed under the children key of their parent.

function list_to_tree($list, $root = 0, $pk = 'id', $pid = 'parent_id', $child = 'children')
{
    $tree = array();
    if (is_array($list)) {
        $refer = array();
        foreach ($list as $key => $data) {
            $refer[$data[$pk]] = &$list[$key];
        }
        foreach ($list as $key => $data) {
            $parentId = 0;
            if (isset($data[$pid])) {
                $parentId = $data[$pid];
            }
            if ((string)$root == $parentId) {
                $tree[] = &$list[$key];
            } else {
                if (isset($refer[$parentId])) {
                    $parent = &$refer[$parentId];
                    $parent[$child][] = &$list[$key];
                }
            }
        }
    }
    return $tree;
}

The tree_to_list function performs the inverse operation: it recursively traverses the tree, removes the children field from each node, and merges the flattened nodes into a single array.

function tree_to_list($tree = [], $children = 'children')
{
    if (empty($tree) || !is_array($tree)) {
        return $tree;
    }
    $arrRes = [];
    foreach ($tree as $k => $v) {
        $arrTmp = $v;
        unset($arrTmp[$children]);
        $arrRes[] = $arrTmp;
        if (!empty($v[$children])) {
            $arrTmp = tree_to_list($v[$children]);
            $arrRes = array_merge($arrRes, $arrTmp);
        }
    }
    return $arrRes;
}

By using these two functions, developers can easily switch between flat and hierarchical representations of data, which is useful for rendering nested menus, category trees, or organizational charts.