I'm writing some little helper classes to handle trees. Basically, I have a node and a special root node that represents the tree. I want to keep it generic and simple. This is part of the code:

<?php

class Tree extends TreeNode{
    public function addById($node_id, $parent_id, $generic_content){
        if( $parent = $this->findNodeById($parent_id) ){
            $parent->addChildById($node_id, $generic_content);
        }
    }
}

class TreeNode{
    public function __construct($node_id, $parent_id, $generic_content){
        // ...
    }

    protected function addChildById($node_id, $generic_content){
        $this->children[] = new TreeNode($this->node_id, $node_id, $generic_content);
    }
}

$Categories = new Tree;
$Categories->addById(1, NULL, $foo);
$Categories->addById(2, NULL, $bar);
$Categories->addById(3, 1, $gee);

?>

My questions:

• Is it sensible to force TreeNode instances to be created through TreeNode::addById()?
• If it's so, would it be good practise to declare TreeNode::__construct() as private/protected?