build tree from array of instances

function buildTree($flat, $pidKey, $idKey = null) {
    $grouped = array();
    foreach ($flat as $sub) {
        $sub->depth = 0;
        $grouped[$sub->{$pidKey}][] = $sub;
    }

    $fnBuilder = function ($siblings, $depth) use (&$fnBuilder, $grouped, $idKey) {
        foreach ($siblings as $k => $sibling) {
            $id = $sibling->{$idKey};
            $sibling->depth = $depth;
            if (isset($grouped[$id])) {
                $sibling->children = $fnBuilder($grouped[$id], $depth + 1);
            }
            $siblings[$k] = $sibling;
        }

        return $siblings;
    };

    return $fnBuilder($grouped[0], 0);
}