Inorder traversal of binary tree using stack in php

Php program for Inorder traversal of binary tree using stack. Here problem description and explanation.

<?php
// Php program for
// Inorder traversal without recursion
// Using stack

// Binary Tree node
class TreeNode
{
	public $data;
	public $left;
	public $right;
	// Make a tree node
	public	function __construct($data)
	{
		// Set node data
		$this->data = $data;
		$this->left = NULL;
		$this->right = NULL;
	}
}
// Stack Class
class MyStack
{
	public $top;
	public $size;
	public	function __construct()
	{
		$this->top = NULL;
		$this->size = 0;
	}
	// Add new element into stack
	public	function push($element)
	{
		// Create new node
		$node = new StackNode($element);
		$node->next = $this->top;
		// new node is new top
		$this->top = $node;
		// Increase the size
		$this->size += 1;
	}
	// Remove top element of the stack
	public	function pop()
	{
		if ($this->top != NULL)
		{
			// next node is new top
			$this->top = $this->top->next;
			// Reduce size
			$this->size -= 1;
		}
	}
	// Returns the status of empty or non empty stacks
	public	function isEmpty()
	{
		if ($this->size > 0 && $this->top != NULL)
		{
			return false;
		}
		else
		{
			return true;
		}
	}
	// Return top element of stack
	public	function peek()
	{
		if ($this->isEmpty())
		{
			return NULL;
		}
		return $this->top->element;
	}
}
class StackNode
{
	public $element;
	public $next;
	public	function __construct($element)
	{
		$this->element = $element;
		$this->next = NULL;
	}
}

class BinaryTree
{
	public $root;
	public	function __construct()
	{
		$this->root = NULL;
	}
	// Display Inorder view of binary tree
	public	function inorder()
	{
		if ($this->root != NULL)
		{
			// Make new stack
			$s = new MyStack();
			$node = $this->root;
			// Display tree element
			while (!$s->isEmpty() || $node != NULL)
			{
				if ($node != NULL)
				{
					// Add current node
					$s->push($node);
					// Visit to left subtree
					$node = $node->left;
				}
				else
				{
					// When node is null
					// Get stack top element
					$node = $s->peek();
					// Display node value
					printf("%s", " ".strval($node->data));
					// Remove top element of the stack
					$s->pop();
					// Visit to left subtree of current node
					$node = $node->right;
				}
			}
		}
		else
		{
			printf("%s", "Empty Linked List\n");
		}
	}
	public static
	function main($args)
	{
		// Create new tree
		$tree = new BinaryTree();
		/*
		    Construct Binary Tree
		    -----------------------
		        15
		       /  \
		      24   54
		     /    /  \
		    35   62   13
		*/
		// Add tree nodes
		$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 result
		$tree->inorder();
	}
}
BinaryTree::main(array());

Output

 35 24 15 62 54 13