Inorder traversal of binary tree with recursion in php

Php program for Inorder traversal of binary tree with recursion. Here problem description and explanation.

<?php
/* 
  Php Program for
  inorder tree traversal of a Binary Tree
  using recursion
*/
// Binary Tree Node
class TreeNode
{
	public $data;
	public $left;
	public $right;
	public	function __construct($data)
	{
		// Set node value
		$this->data = $data;
		$this->left = NULL;
		$this->right = NULL;
	}
}
class BinaryTree
{
	public $root;
	public	function __construct()
	{
		$this->root = NULL;
	}
	// Display Inorder view of binary tree
	public	function inorder($node)
	{
		if ($node != NULL)
		{
			// Visit left subtree
			$this->inorder($node->left);
			//Print node value
			printf("%d  ",$node->data);
			// Visit right subtree
			$this->inorder($node->right);
		}
	}
	public static
	function main($args)
	{
		// Create new tree
		$tree = new BinaryTree();
		/*
		    Make A Binary Tree
		    ----------------
		        15
		       /  \
		      24   54
		     /    /  \
		    35   62   13
		*/
		// Add tree TreeNode
		$tree->root = new TreeNode(15);
		$tree->root->left = new TreeNode(24);
		$tree->root->right = new TreeNode(54);
		$tree->root->right->right = new TreeNode(13);
		$tree->root->right->left = new TreeNode(62);
		$tree->root->left->left = new TreeNode(35);
		// Display Tree Node
		$tree->inorder($tree->root);
	}
}
BinaryTree::main(array());

Output

35  24  15  62  54  13