Reverse spiral traversal of a binary tree in php

Php program for Reverse spiral traversal of a binary tree. Here problem description and other solutions.

<?php
/*
  Php program for
  Reverse level order traversal in spiral form
*/
// 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;
	}
}
// Stack node
class StackNode
{
	// Stack data
	public $element;
	public $next;
	public	function __construct($element, $top)
	{
		$this->element = $element;
		$this->next = $top;
	}
}
// Define a custom stack
class MyStack
{
	public $top;
	public $size;
	public	function __construct()
	{
		$this->top = NULL;
		$this->size = 0;
	}
	// Add node at the top of stack
	public	function push($element)
	{
		$this->top = new StackNode($element, $this->top);
		$this->size++;
	}
	public	function isEmpty()
	{
		if ($this->size > 0 && $this->top != NULL)
		{
			return false;
		}
		else
		{
			return true;
		}
	}
	// Remove top element of stack
	public	function pop()
	{
		if ($this->size > 0 && $this->top != NULL)
		{
			// Change top element of stack
			$this->top = $this->top->next;
			$this->size--;
		}
	}
	// Return top element of stack
	public	function peek()
	{
		if ($this->size == 0)
		{
			return NULL;
		}
		return $this->top->element;
	}
}
class BinaryTree
{
	public $root;
	public	function __construct()
	{
		$this->root = NULL;
	}
	// Display tree element in reverse spiral level order traversal
	public	function reverseSpiral()
	{
		if ($this->root == NULL)
		{
			// Case
			// When empty
			echo "Empty Tree\n";
			return;
		}
		// Empty stack
		$s1 = new MyStack();
		$s2 = new MyStack();
		$result = new MyStack();
		// Some auxiliary variable
		$status = 1;
		$node = $this->root;
		// Add first node
		$s1->push($node);
		while ($node != NULL)
		{
			// Add node element into resultant stack
			$result->push($node);
			if ($status == 1)
			{
				// Add node from right to left
				// in s2 stack
				if ($node->right != NULL)
				{
					// Add right child
					$s2->push($node->right);
				}
				if ($node->left != NULL)
				{
					// Add left child
					$s2->push($node->left);
				}
			}
			else
			{
				// Add node from left to right
				// in s1 stack
				if ($node->left != NULL)
				{
					// Add left child
					$s1->push($node->left);
				}
				if ($node->right != NULL)
				{
					// Add right child
					$s1->push($node->right);
				}
			}
			if ($status == 1)
			{
				// Case A
				// When execute s1 stack
				// Remove s1 element
				$s1->pop();
				if ($s1->isEmpty())
				{
					// When after remove s1 element
					// s1 stack empty.
					// Then change stack by s2
					$status = 2;
					// Get first element of s2
					$node = $s2->peek();
				}
				else
				{
					// Otherwise get new top
					$node = $s1->peek();
				}
			}
			else
			{
				// Case B
				// When execute s2 stack
				// Remove s2 element
				$s2->pop();
				if ($s2->isEmpty())
				{
					// Here change stack
					$status = 1;
					$node = $s1->peek();
				}
				else
				{
					$node = $s2->peek();
				}
			}
		}
		// Display final result
		while ($result->isEmpty() == false)
		{
			// Get top element
			$node = $result->peek();
			// Display node value
			echo "   ".strval($node->data);
			// Remove top of stack
			$result->pop();
		}
	}
	public static function main()
	{
		// Create new tree
		$tree = new BinaryTree();
		/*   Make A Binary Tree
		    ---------------
		       1
		      / \ 
		     /   \
		    2     3
		   /     / \
		  4     5   6
		   \    \    \
		    7    8    9
		        
		*/
		// Add node
		$tree->root = new TreeNode(1);
		$tree->root->left = new TreeNode(2);
		$tree->root->right = new TreeNode(3);
		$tree->root->right->right = new TreeNode(6);
		$tree->root->right->left = new TreeNode(5);
		$tree->root->left->left = new TreeNode(4);
		$tree->root->left->left->right = new TreeNode(7);
		$tree->root->right->left->right = new TreeNode(8);
		$tree->root->right->right->right = new TreeNode(9);
		// Display reverse spiral level order element
		$tree->reverseSpiral();
	}
}
BinaryTree::main();

Output

   9   8   7   4   5   6   3   2   1