<?php
/*
 * node.value
 * node.left
 * node.right
 */

class Node {
    public $value;
    public $left;
    public $right;

    public function __construct($value)
    {
        $this->value = $value;
    }

    public function add(Node $node, $position = null)
    {
        if ($position == 'l') {
            $this->left = &$node;
            return $node;
        } else {
            $this->right = &$node;
            return $node;
        }
    }
}

$node = new Node(1);
$left = $node->add(new Node(2), 'l');
$right = $node->add(new Node(3));
$right->add(new Node(4), 'l');
$right->add(new Node(5));
$left->add(new Node(6), 'l');
$left->add(new Node(7));


inorder($node);

function inorder(&$node)
{
    if($node == null) return null;

    inorder($node->left);
    echo $node->value . "<br />";
    inorder($node->right);
}