trash.php

<?php
...
    public static function tree()
    {
        function makeTree(&$sectionBranch, $sectionArray)
        {
            foreach ($sectionBranch as $k => $sectionLeave) {
                $sectionLeave->children = array_values(array_filter($sectionArray, function ($item) use ($sectionLeave) {
                    return $item->parent_id == $sectionLeave->id;
                }));
                makeTree($sectionLeave->children, $sectionArray);
                $sectionBranch[$k] = $sectionLeave;
            }
        };
        $sectionArray = array_map(function ($section) {
            return (object) $section;
        }, self::with(['items' => function ($query) {
            $query->select('id', 'section_id');
        }, 'recomendation'])->ordered()->toArray());

        $sectionTree = [(object) ['id' => null, 'caption' => 'root']];
        makeTree($sectionTree, $sectionArray);
        return $sectionTree[0]->children;
    }
?>