Sum of all the grandchild nodes of binary tree in php

Php program for Sum of all the grandchild nodes of binary tree. Here mentioned other language solution.

<?php
/*
    Php program for
    Sum of nodes in binary tree whose grandparents exists
*/
// 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()
	{
		$this->root = NULL;
	}
	public	function preOrder($node)
	{
		if ($node != NULL)
		{
			// Print node value
			echo "  ".($node->data);
			// Visit left subtree
			$this->preOrder($node->left);
			// Visit right subtree
			$this->preOrder($node->right);
		}
	}
	public	function grandchildSum($node, $level)
	{
		if ($node != NULL)
		{
			$result = 0;
			if ($level > 2)
			{
				// Get the node
				$result = $node->data;
			}
			// Visit left subtree and right subtree
			return $result + 
              $this->grandchildSum($node->left, $level + 1) + 
              $this->grandchildSum($node->right, $level + 1);
		}
		else
		{
			return 0;
		}
	}
	public static function main()
	{
		// Create new binary trees
		$tree = new BinaryTree();
		/*
		         4   
		        /  \                          
		       /    \    
		      12     7    
		     / \      \               
		    2   3      1
		       / \    / 
		      6   8  5
		     /        
		    9                        
		  ----------------
		  Constructing binary tree    
		*/
		// Add nodes
		$tree->root = new TreeNode(4);
		$tree->root->left = new TreeNode(12);
		$tree->root->left->right = new TreeNode(3);
		$tree->root->left->right->left = new TreeNode(6);
		$tree->root->left->right->left->left = new TreeNode(9);
		$tree->root->left->right->right = new TreeNode(8);
		$tree->root->left->left = new TreeNode(2);
		$tree->root->right = new TreeNode(7);
		$tree->root->right->right = new TreeNode(1);
		$tree->root->right->right->left = new TreeNode(5);
		// Display tree node
		echo "\n Tree Nodes ";
		$tree->preOrder($tree->root);
		// Find node sum
		// 2 + 3 + 6 + 9 + 8 + 1 + 5 = 34
		$result = $tree->grandchildSum($tree->root, 1);
		// Display result
		echo "\n Result : ".($result), "\n";
	}
}
BinaryTree::main();

Output

 Tree Nodes   4  12  2  3  6  9  8  7  1  5
 Result : 34