How to Recursively Build a Multidimensional Array from Flat Database Results?

Building a Multidimensional Array from Database Results Recursively

To retrieve hierarchical data structures, such as page and category menus, from a flat database result, a recursive function can be employed. This function will take the original array and organize it into nested arrays based on parent-child relationships.

The Function:

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;
}

How it Works:

1. The function takes in an array of elements and an optional parent ID, defaulting to 0.
2. It iterates over each element in the array.
3. If an element's parent ID matches the specified parent ID, it is considered a child of that parent.
4. The function recursively calls itself to find all children of the current child element.
5. If the element has any children, they are added as an array under the 'children' key of the current element.
6. Finally, the function returns the array of child elements for the given parent.

Example Usage:

To convert the example database result into a multidimensional array using this function:

$tree = buildTree($rows);

Where $rows is the original array of database results.

Output:

The resulting hierarchical array will resemble the desired output:

Array
(
    [0] => Array
        (
            [id] => 1
            [parent_id] => 0
            [title] => Parent Page
            [children] => Array
                        (
                            [0] => Array
                                (
                                    [id] => 2
                                    [parent_id] => 1
                                    [title] => Sub Page
                                    [children] => Array
                                                (
                                                    [0] => Array
                                                        (
                                                            [id] => 3
                                                            [parent_id] => 1
                                                            [title] => Sub Sub Page
                                                        )
                                                )
                                )
                        )
            [children] => Array
                        (
                            [0] => Array
                                (
                                    [id] => 4
                                    [parent_id] => 0
                                    [title] => Another Parent Page
                                    [children] => Array
                                                (

                                                )
                                )
                        )
        )
)

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