categories

  $categories = [
      [
          'name' => 'cat1',
          'children' => [
                  [
                  'name' => 'cat1',
                  'children' => [
                      [
                          'name'=>'subcat1',
                          'children'=> []
                      ]
                  ]
              ]
          ]
      ]
  ]
  
  function list_categories(Array $categories)
{
  $data = [];

  foreach($categories as $category)
  {
    $data[] = [
      'name' = $category->name,
      'children' = list_categories($category->children),
    ];
  }

  return $data;
}

$array_categories = list_categories($root_categories);




Example for tree categories

<?php

use Illuminate\Database\Eloquent\Collection;

class Tree extends Collection {

    /**
     * Build tree structure from the collection.
     * 
     * @return static
     */
    public function buildTree()
    {
        $structure = $this->matchNodes();

        $this->clean($structure);

        return $structure;
    }

    /**
     * Match child nodes with the parents.
     * 
     * @return static
     */
    protected function matchNodes()
    {
        $structure = new static($this->items);

        foreach ($this->items as $key => $node)
        {
            $parentId = $node->getParentId();
            $nodeId   = $node->getKey();

            if ($parentId)
            {
                $parent = $structure->find($parentId)
                    ->subtree
                    ->add($node);
            }
        }

        return $structure;
    }

    /**
     * Clean root level of the structure.
     *
     * @param  static $structure
     * @return void
     */
    protected function clean($structure)
    {
        foreach ($structure as $key => $node)
        {
            if ($node->getParentId())
            {
                $structure->forget($key);
            }
        }
    }
...
}
<?php

class Node extends \Eloquent { // might be used as a trait as well

    protected $subtree;

    public function getSubtreeAttribute()
    {
        return $this->subtree = $this->subtree ?: $this->newCollection();
    }

    public function newCollection(array $models = array())
    {
        return new Tree($models);
    }
...
}
Then all you need is this:

$tree = Category::all()->buildTree();
$tree->toArray();
// nice tree structure of your category objects