Convert a Flat Array into a Nested Array Using Recursion

Problem: How to Convert a Flat Array into a Nested Array Using Recursion in PHP?

I have a flat array with items that have parent-child relationships. Each item has an ID and a parent_id. I want to convert this flat array into a nested array where each parent contains its children recursively. How can I achieve this in PHP?

Here is an example of the flat array:

$items = [
    ['id' => 1, 'parent_id' => 0, 'name' => 'Parent 1'],
    ['id' => 2, 'parent_id' => 1, 'name' => 'Child 1.1'],
    ['id' => 3, 'parent_id' => 1, 'name' => 'Child 1.2'],
    ['id' => 4, 'parent_id' => 2, 'name' => 'Child 1.1.1'],
    ['id' => 5, 'parent_id' => 0, 'name' => 'Parent 2'],
    ['id' => 6, 'parent_id' => 5, 'name' => 'Child 2.1']
];

I want to transform it into a nested structure like this:

[
    [
        'id' => 1,
        'parent_id' => 0,
        'name' => 'Parent 1',
        'children' => [
            [
                'id' => 2,
                'parent_id' => 1,
                'name' => 'Child 1.1',
                'children' => [
                    [
                        'id' => 4,
                        'parent_id' => 2,
                        'name' => 'Child 1.1.1'
                    ]
                ]
            ],
            [
                'id' => 3,
                'parent_id' => 1,
                'name' => 'Child 1.2'
            ]
        ]
    ],
    [
        'id' => 5,
        'parent_id' => 0,
        'name' => 'Parent 2',
        'children' => [
            [
                'id' => 6,
                'parent_id' => 5,
                'name' => 'Child 2.1'
            ]
        ]
    ]
]

How can I do this using a recursive function in PHP?

Solution: Creating a Nested Array Using a Recursive Function in PHP

To achieve this, you need a recursive function that builds the tree structure by looking for children of each item. The function will group items by their parent_id and attach them to their corresponding parents.

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Step-by-Step Implementation:

First, we create a function that takes the flat array and the parent_id to find children recursively.
We then loop through the array and attach child nodes to their respective parents.

Here’s how to do it:

function buildTree(array $items, $parentId = 0) {
    $branch = [];

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

    return $branch;
}

// Example flat array
$items = [
    ['id' => 1, 'parent_id' => 0, 'name' => 'Parent 1'],
    ['id' => 2, 'parent_id' => 1, 'name' => 'Child 1.1'],
    ['id' => 3, 'parent_id' => 1, 'name' => 'Child 1.2'],
    ['id' => 4, 'parent_id' => 2, 'name' => 'Child 1.1.1'],
    ['id' => 5, 'parent_id' => 0, 'name' => 'Parent 2'],
    ['id' => 6, 'parent_id' => 5, 'name' => 'Child 2.1']
];

// Convert flat array to nested array
$nestedArray = buildTree($items);

echo '<pre>';
print_r($nestedArray);
echo '</pre>';

Explanation:

The buildTree() function takes two parameters: the flat array and the parentId it should start from (default is 0, representing the top-level items).
It loops through the items to check which ones have the matching parent_id.
For each item that matches, it recursively calls buildTree() to check if that item has children. If it does, it adds them under a children key.
The function returns the built tree for the given parent_id.

Output:

The resulting nested array will be:

Array
(
    [0] => Array
        (
            [id] => 1
            [parent_id] => 0
            [name] => Parent 1
            [children] => Array
                (
                    [0] => Array
                        (
                            [id] => 2
                            [parent_id] => 1
                            [name] => Child 1.1
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [id] => 4
                                            [parent_id] => 2
                                            [name] => Child 1.1.1
                                        )

                                )

                        )

                    [1] => Array
                        (
                            [id] => 3
                            [parent_id] => 1
                            [name] => Child 1.2
                        )

                )

        )

    [1] => Array
        (
            [id] => 5
            [parent_id] => 0
            [name] => Parent 2
            [children] => Array
                (
                    [0] => Array
                        (
                            [id] => 6
                            [parent_id] => 5
                            [name] => Child 2.1
                        )

                )

        )

)

Conclusion:

Using a recursive function to build a tree structure from a flat array is a powerful technique in PHP. It’s particularly useful when dealing with hierarchical data like categories, menus, or organizational charts. By leveraging recursion, you can easily create a nested array where each parent contains its respective children.