Insertion sort on doubly linked list in php

Php program for Insertion sort on doubly linked list. Here problem description and explanation.

<?php
// Php program for
// Perform insertion sort on doubly linked list
class LinkNode
{
	public $data;
	public $next;
	public $prev;
	public	function __construct($data)
	{
		$this->data = $data;
		$this->next = NULL;
		$this->prev = NULL;
	}
}
class DoublyLinkedList
{
	public $head;
	public $tail;
	public	function __construct()
	{
		$this->head = NULL;
		$this->tail = NULL;
	}
	// Insert new node at end position
	public	function insert($value)
	{
		// Create a node
		$node = new LinkNode($value);
		if ($this->head == NULL)
		{
			// Add first node
			$this->head = $node;
			$this->tail = $node;
			return;
		}
		// Add node at last position
		$this->tail->next = $node;
		$node->prev = $this->tail;
		$this->tail = $node;
	}
	// Display node element of doubly linked list
	public	function display()
	{
		if ($this->head == NULL)
		{
			printf("Empty Linked List\n");
		}
		else
		{
			printf("\nLinked List Head to Tail : ");
			// Get first node of linked list
			$temp = $this->head;
			// iterate linked list 
			while ($temp != NULL)
			{
				// Display node value
				printf(" %d ",$temp->data);
				// Visit to next node
				$temp = $temp->next;
			}
			printf("\nLinked List Tail to Head : ");
			// Get last node of linked list
			$temp = $this->tail;
			// iterate linked list 
			while ($temp != NULL)
			{
				// Display node value
				printf(" %d ",$temp->data);
				// Visit to prev node
				$temp = $temp->prev;
			}
			printf("\n");
		}
	}
	// Swap value of given node
	public	function swapData($first, $second)
	{
		$value = $first->data;
		$first->data = $second->data;
		$second->data = $value;
	}
	// Sort elements using insertion sort
	public	function insertionSort()
	{
		// Get first node
		$front = $this->head;
		$back = NULL;
		while ($front != NULL)
		{
			// Get next node
			$back = $front->next;
			// Update node value when consecutive nodes are not sort
			while ($back != NULL && 
                   $back->prev != NULL && 
                   $back->data < $back->prev->data)
			{
				// Modified node data
				$this->swapData($back, $back->prev);
				// Visit to previous node
				$back = $back->prev;
			}
			// Visit to next node
			$front = $front->next;
		}
	}
	public static
	function main($args)
	{
		$dll = new DoublyLinkedList();
		//Insert element of linked list
		$dll->insert(25);
		$dll->insert(2);
		$dll->insert(6);
		$dll->insert(14);
		$dll->insert(12);
		$dll->insert(9);
		$dll->insert(16);
		$dll->insert(3);
		printf("\n Before Sort");
		$dll->display();
		// Display all node
		$dll->insertionSort();
		printf("\n After Sort");
		$dll->display();
	}
}

DoublyLinkedList::main(array());

Output

 Before Sort
Linked List Head to Tail :  25  2  6  14  12  9  16  3
Linked List Tail to Head :  3  16  9  12  14  6  2  25

 After Sort
Linked List Head to Tail :  2  3  6  9  12  14  16  25
Linked List Tail to Head :  25  16  14  12  9  6  3  2