Find kth smallest element in BST php

Php program for Find kth smallest element in BST. Here mentioned other language solution.

<?php
// Php program for
// K’th smallest element in Binary Search Tree
class TreeNode
{
	public $data;
	public $left;
	public $right;
	public	function __construct($data)
	{
		$this->data = $data;
		$this->left = NULL;
		$this->right = NULL;
	}
}
class BinarySearchTree
{
	public $root;
	private $counter;
	private $element;
	public	function __construct()
	{
		$this->root = NULL;
	}
	// Insert a node element
	public	function insert($node, $data)
	{
		if ($node != NULL)
		{
			if ($node->data >= $data)
			{
				// When new element is smaller or equal to current node
				$node->left = $this->insert($node->left, $data);
			}
			else
			{
				// When new element is higher to current node
				$node->right = $this->insert($node->right, $data);
			}
			// Important to manage new node
			return $node;
		}
		else
		{
			return new TreeNode($data);
		}
	}
	// Find the kth smallest node in BST
	public	function findKthSmallest($node, $k)
	{
		if ($node != NULL)
		{
			// Visit left subtree
			$this->findKthSmallest($node->left, $k);
			if ($this->counter == $k)
			{
				// Stop recursion
				return;
			}
			if ($this->element == NULL || 
                $this->element->data < $node->data)
			{
				// Modified counter variable by one
				$this->counter++;
				// Collect possible result
				$this->element = $node;
			}
			// Visit right subtree
			$this->findKthSmallest($node->right, $k);
		}
	}
	public	function printKthSsmallest($k)
	{
		if ($k <= 0)
		{
			// When given k is not valid positive values
			echo "Invalid node position ".strval($k), "\n";
		}
		else if ($this->root == NULL)
		{
			// When BST are no have elements
			echo "Empty Binary Search Tree\n";
		}
		else
		{
			// Reset values
			$this->counter = 0;
			$this->element = NULL;
			// Find result
			$this->findKthSmallest($this->root, $k);
			if ($this->counter < $k)
			{
				// If In case given kth smallest node are
				// Not exist in binary search tree, then
				echo "Given ".strval($k)
				." smallest node are not exist "
                . strval($this->counter), "\n";
			}
			else
			{
				echo "[".strval($k).
				"] smallest node : "
                .strval($this->element->data), "\n";
			}
		}
	}
	// Handle request to add new node
	public	function addNode($key)
	{
		$this->root = $this->insert($this->root, $key);
	}
	public static function main()
	{
		$tree = new BinarySearchTree();
		/*
		    Make Tree
		    ----------
		        7
		       / \ 
		      /   \
		     4     9
		    / \   / \
		   3   5 8   10
		      /    
		     4     
		*/
		// Add node
		$tree->addNode(7);
		$tree->addNode(4);
		$tree->addNode(3);
		$tree->addNode(5);
		$tree->addNode(9);
		$tree->addNode(8);
		$tree->addNode(10);
		$tree->addNode(4);
		// Testing
		$tree->printKthSsmallest(6);
		$tree->printKthSsmallest(4);
		$tree->printKthSsmallest(3);
	}
}
BinarySearchTree::main();

Output

[6] smallest node : 9
[4] smallest node : 7
[3] smallest node : 5