Using a Recursive Function to Format Flat Arrays

array.php

<?php

$flat_array = [

  [
    'parent' => 0,
    'id'    => 1,
    'title' => 'Parent 1',
    'content' => [
       'c12',
       'c13'
    ],
    'children' => [
       [
        'parent' => 1,
        'id'    => 'c12',
        'title' => 'Child 1',
        'content' => [
           'c21'
        ],
        'children' => [
           [
            'parent' => 'c12',
            'id'    => 'c21',
            'title' => 'Sub Child 2'
          ]
        ],
 
      ],
      [
        'parent' => 1,
        'id'    => 'c13',
        'title' => 'Child 2'
      ],
    ],
  ],
 
]

flat-array.php

<?php

$flat_array = [

  [
    'parent' => 0,
    'id'    => 1,
    'title' => 'Parent 1',
    'content' => [
       'c12',
       'c13'
    ]
  ],
  [
    'parent' => 1,
    'id'    => 'c12',
    'title' => 'Child 1',
    'content' => [
       'c21'
    ]
  ],
  [
    'parent' => 1,
    'id'    => 'c13',
    'title' => 'Child 2'
  ],
  [
    'parent' => 'c12',
    'id'    => 'c21',
    'title' => 'Sub Child 2'
  ]
]

recursive.php

<?php

function unFlattenArray( $arrayItems, $parentId = 0 ) {
  $branch = array();
  foreach ($arrayItems as $item) {
      if ( $parentId === $item['parent'] ) {
          $childrenItems = unFlattenArray( $arrayItems, $item['id'] );
          if ( $childrenItems ) {
              $item['children'] = [];
     
              foreach ( $childrenItems as $childItem ) {
                  $item['children'][] = $childItem;
              }
          }
          
          $branch[] = $item;
      }
  }
  return $branch;
}