Check palindrome in a doubly linked list in php

Php program for Detect palindrome in doubly linked list. Here more information.

<?php
// Php program for
// Check if palindrome exists in doubly linked list

// This is Linked List Node
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)
		{
			echo "Empty Linked List\n";
		}
		else
		{
			echo "Linked List Head to Tail :";
			// Get first node of linked list
			$temp = $this->head;
			// iterate linked list 
			while ($temp != NULL)
			{
				// Display node value
				echo "  ",$temp->data;
				// Visit to next node
				$temp = $temp->next;
			}
			echo "\nLinked List Tail to Head :";
			// Get last node of linked list
			$temp = $this->tail;
			// iterate linked list 
			while ($temp != NULL)
			{
				// Display node value
				echo "  ",$temp->data;
				// Visit to prev node
				$temp = $temp->prev;
			}
		}
	}
	public	function isPalindrome()
	{
		if ($this->head == NULL)
		{
			// When empty linked list
			return false;
		}
		$first = $this->head;
		$last = $this->tail;
		while ($last != NULL && 
               $first != NULL && 
               $first != $last)
		{
			if ($first->data != $last->data)
			{
				// When node value are not same
				return false;
			}
			if ($first->next == $last)
			{
				// When get last pair
				if ($first->data == $last->data)
				{
					// When pair value is same
					return true;
				}
				// When last middle node not same
				return false;
			}
			// Visit to next node
			$first = $first->next;
			// Visit to previous node
			$last = $last->prev;
		}
		return true;
	}
	public static
	function main($args)
	{
		$dll = new DoublyLinkedList();
		// Insert following linked list nodes
		$dll->insert(1);
		$dll->insert(2);
		$dll->insert(4);
		$dll->insert(3);
		$dll->insert(4);
		$dll->insert(2);
		$dll->insert(1);
		// Display all node
		$dll->display();
		// Test A
		if ($dll->isPalindrome() == true)
		{
			echo "\nYes\n";
		}
		else
		{
			echo "\nNo\n";
		}
		// Add new element
		$dll->insert(1);
		$dll->display();
		// Test B
		if ($dll->isPalindrome() == true)
		{
			echo "\nYes\n";
		}
		else
		{
			echo "\nNo\n";
		}
	}
}
DoublyLinkedList::main(array());

Output

Linked List Head to Tail :  1  2  4  3  4  2  1
Linked List Tail to Head :  1  2  4  3  4  2  1
Yes
Linked List Head to Tail :  1  2  4  3  4  2  1  1
Linked List Tail to Head :  1  1  2  4  3  4  2  1
No