Check for foldable binary tree in php

Php program for Check for foldable binary tree. Here problem description and explanation.

<?php
/* 
  Php program for
  Check if binary tree is foldable 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()
	{
		$this->root = NULL;
	}
	public	function isFoldable($leftRoot, 
                                 $rightRoot)
	{
		if ($leftRoot == NULL && $rightRoot == NULL)
		{
			// When both node empty
			return true;
		}
		else if ($leftRoot != NULL && $rightRoot != NULL && 
                 $this->isFoldable($leftRoot->left, $rightRoot->right) && 
                 $this->isFoldable($leftRoot->right, $rightRoot->left))
		{
			// When
			// Valid the properties of foldable tree
			return true;
		}
		// Fail case
		return false;
	}
	// Display tree element inorder form
	public	function inorder($node)
	{
		if ($node != NULL)
		{
			// Visit to left subtree
			$this->inorder($node->left);
			// Print node value
			echo "  $node->data";
			// Visit to right subtree
			$this->inorder($node->right);
		}
	}
	public static function main()
	{
		// Create new tree
		$tree = new BinaryTree();
		/*
		   Binary Tree
		  -----------------------
		       5
		     /   \
		    7     4
		   /       \
		  1         2
		   \       /
		    2     5
		*/
		// Binary tree nodes
		$tree->root = new TreeNode(5);
		$tree->root->left = new TreeNode(7);
		$tree->root->right = new TreeNode(4);
		$tree->root->left->left = new TreeNode(1);
		$tree->root->right->right = new TreeNode(2);
		$tree->root->right->right->left = new TreeNode(5);
		$tree->root->left->left->right = new TreeNode(2);
		// Display Tree elements
		$tree->inorder($tree->root);
		$result = $tree->isFoldable($tree->root->left, 
                                    $tree->root->right);
		if ($result == true)
		{
			echo "\n Foldable Tree\n";
		}
		else
		{
			echo "\n Not Foldable Tree\n";
		}
		/*
		         5
		       /   \
		      7     4
		     /       \
		    1         2
		     \       /
		      2     5
		       \
		        3  
		*/
		// Case 2
		$tree->root->left->left->right->right = new TreeNode(3);
		$tree->inorder($tree->root);
		$result = $tree->isFoldable($tree->root->left, 
                                    $tree->root->right);
		if ($result == true)
		{
			echo "\n Foldable Tree\n";
		}
		else
		{
			echo "\n Not Foldable Tree\n";
		}
	}
}
BinaryTree::main();

Output

  1  2  7  5  4  5  2
 Foldable Tree
  1  2  3  7  5  4  5  2
 Not Foldable Tree