Flat array to Multi-level Tree

index.php

<?php

$data = [
  [
    'id' => 1,
    'name' => 'A',
    'parent_id' => null,
  ],
  [
    'id' => 2,
    'name' => 'B',
    'parent_id' => 1,
  ],
  [
    'id' => 3,
    'name' => 'C',
    'parent_id' => 1,
  ],
  [
    'id' => 4,
    'name' => 'D',
    'parent_id' => 2,
  ],
  [
    'id' => 5,
    'name' => 'E',
    'parent_id' => 2,
  ],
  [
    'id' => 6,
    'name' => 'F',
    'parent_id' => 4,
  ],
  [
    'id' => 7,
    'name' => 'G',
    'parent_id' => 4,
  ],
  [
    'id' => 8,
    'name' => 'H',
    'parent_id' => null,
  ],
  [
    'id' => 9,
    'name' => 'I',
    'parent_id' => 8,
  ],
  [
    'id' => 10,
    'name' => 'J',
    'parent_id' => 9,
  ],
];

function buildTree( array $elements, $parent_id = null, $id_key = 'id', $parent_key = 'parent_id', $children_key = 'children' )
{
    $branch = [];

    foreach ( $elements as $element )
    {
        if ( $element[ $parent_key ] === $parent_id )
        {
            $children = buildTree( $elements, $element[ $id_key ], $id_key, $parent_key, $children_key );

            if ( ! empty( $children ) ) {
                $element[ $children_key ] = $children;
            }

            $branch[] = $element;
        }
    }

    return $branch;
}

var_dump( buildTree( $data ) );