Move vowels node at the beginning of linked list in php

Php program for Move vowels node at the beginning of linked list. Here problem description and other solutions.

<?php
/*
  Php program for
  Shift the vowels node at beginning of linked list
*/
// Linked list node
class LinkNode
{
	public $data;
	public $next;
	public	function __construct($data)
	{
		$this->data = $data;
		$this->next = NULL;
	}
}
class SingleLL
{
	public $head;
	public $tail;
	public	function __construct()
	{
		$this->head = NULL;
		$this->tail = NULL;
	}
	// Add new node at end of linked list 
	public	function addNode($data)
	{
		$node = new LinkNode($data);
		if ($this->head == NULL)
		{
			$this->head = $node;
		}
		else
		{
			// Append the node at last position
			$this->tail->next = $node;
		}
		$this->tail = $node;
	}
	// Display linked list element
	public	function display()
	{
		if ($this->head == NULL)
		{
			echo "\n Empty linked list\n";
			return;
		}
		$temp = $this->head;
		// iterating linked list elements
		while ($temp != NULL)
		{
			echo " ".strval($temp->data).
			" →";
			// Visit to next node
			$temp = $temp->next;
		}
		echo " NULL\n";
	}
	// Determine that given value is an vowel or not
	public	function isVowels($value)
	{
		if ($value == 'a' || $value == 'e' || 
            $value == 'i' || $value == 'o' || 
            $value == 'u' || $value == 'A' || 
            $value == 'E' || $value == 'I' || 
            $value == 'O' || $value == 'U')
		{
			return true;
		}
		return false;
	}
	// Move the vowels value at beginning position
	public	function shiftVowels()
	{
		if ($this->head == NULL)
		{
			echo "\n Empty linked list\n";
			return;
		}
		// Define some auxiliary variable
		$node = $this->head;
		$back = NULL;
		// iterate linked list nodes
		// And shift vowels at front of linked list
		while ($node != NULL)
		{
			if ($this->isVowels($node->data) == true && 
                $back != NULL)
			{
				// Enter here when
				// Node is contain vowel and 
				// its previous (any) node is not form of vowel
				if ($node == $this->tail)
				{
					// When is last node
					// Set new last node
					$this->tail = $back;
				}
				// Connect previous node to next node
				$back->next = $node->next;
				// Connect vowel node to first node
				$node->next = $this->head;
				// Set new head
				$this->head = $node;
				// Get back node
				$node = $back;
			}
			else
			{
				$back = $node;
			}
			// Visit to next node
			$node = $node->next;
		}
	}
	public static
	function main($args)
	{
		$sll = new SingleLL();
		// Constructed linked list
		$sll->addNode('E');
		$sll->addNode('D');
		$sll->addNode('U');
		$sll->addNode('C');
		$sll->addNode('A');
		$sll->addNode('T');
		$sll->addNode('I');
		$sll->addNode('O');
		$sll->addNode('N');
		echo " Before move vowels", "\n";
		// E → D → U → C → A → T → I → O → N → NULL
		$sll->display();
		$sll->shiftVowels();
		echo " After move vowels", "\n";
		// O → I → A → U → E → D → C → T → N → NULL
		$sll->display();
	}
}
SingleLL::main(array());

Output

 Before move vowels
 E → D → U → C → A → T → I → O → N → NULL
 After move vowels
 O → I → A → U → E → D → C → T → N → NULL