Рекурсивное отображение категорий

gistfile1.php

function getCategories($categories, &$result, $parent_id = 0, $depth = 0)
{
    //filter only categories under current "parent"
    $cats = $categories->filter(function ($item) use ($parent_id) {
        return $item->parent_id == $parent_id;
    });

    //loop through them
    foreach ($cats as $cat)
    {
        //add category. Don't forget the dashes in front. Use ID as index
        $result[$cat->id] = str_repeat('-', $depth) . $cat->title;
        //go deeper - let's look for "children" of current category
        getCategories($categories, $result, $cat->id, $depth + 1);
    }
}

//get categories data. In this case it's eloquent.
$categories = Category::get();
//if you don't have the eloquent model you can use DB query builder:
//$categories = DB::table('categories')->select('id', 'parent_id', 'title')->get();
//prepare an empty array for $id => $formattedVal storing
$result = [];
//start by root categories
getCategories($categories, $result);