How Can a Recursive Function Create a Multidimensional Array from a Flat Database Result?

Recursive Function to Generate Multidimensional Array from Database Result

Problem:

Building nested arrays hierarchically from a flat result table, where each row represents an item with its parent ID.

Solution:

function buildTree(array $elements, $parentId = 0) {
    $branch = array();

    foreach ($elements as $element) {
        if ($element['parent_id'] == $parentId) {
            $children = buildTree($elements, $element['id']);
            if ($children) {
                $element['children'] = $children;
            }
            $branch[] = $element;
        }
    }

    return $branch;
}

$tree = buildTree($rows);

Explanation:

The algorithm iteratively builds a hierarchical tree structure using recursion:

1. It initializes an empty array $branch to store the current level of the tree.
2. It iterates through the elements and checks if an element's parent_id matches the specified $parentId.
3. If a match is found, it means the element is a child of the current parent and should be added to the $branch.
4. It recursively calls the buildTree function with the element's id as the new $parentId to find its children.
5. If any children are found, they are added as a sub-array to the current element.
6. After processing all children, the $branch array is returned to represent the next level of the hierarchy.
7. The initial call to buildTree with an initial $parentId of 0 fetches the root elements of the hierarchy, and the recursive calls populate the nested structure.

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