Arrange consonants and vowels nodes in a linked list in php

Php program for Arrange consonants and vowels nodes in a linked list. Here problem description and explanation.

<?php
/*
  Php program for 
  Arrange consonants and vowels nodes in a 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;
		$this->tail = NULL;
	}
	// Add new Node at end of linked list 
	public	function appendNode($node)
	{
		if ($this->head == NULL)
		{
			$this->head = $node;
		}
		else
		{
			// Append the node at last position
			$this->tail->next = $node;
		}
		$this->tail = $node;
	}
	// Handles the request of adding new node in linked list
	public	function addNode($data)
	{
		$node = new LinkNode($data);
		$this->appendNode($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 " $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' || $value == 'U')
		{
			return true;
		}
		return false;
	}
	// Arrange the nodes in front position of vowels 
	// and end with consonants
	public	function arrangeNodes()
	{
		if ($this->head == NULL)
		{
			echo "\n Empty linked list\n";
			return;
		}
		// Define some auxiliary variable
		$vowel = new SingleLL();
		$consonant = new SingleLL();
		$auxiliary = $this->head;
		$ordinary = NULL;
		// iterate linked list nodes
		while ($auxiliary != NULL)
		{
			$ordinary = $auxiliary;
			$auxiliary = $auxiliary->next;
			if ($this->isVowels($ordinary->data) == true)
			{
				// Add node into volwel list
				$vowel->appendNode($ordinary);
			}
			else
			{
				$consonant->appendNode($ordinary);
			}
			$ordinary->next = NULL;
		}
		if ($vowel->head != NULL)
		{
			// Set new head node of resultant linked list
			$this->head = $vowel->head;
			// Connect vowel and consonant linked list
			$vowel->tail->next = $consonant->head;
			if ($consonant->tail != NULL)
			{
				// When consonant node exists
				// Set new tail node of resultant linked list 
				$this->tail = $consonant->tail;
			}
			else
			{
				// When consonant nodes not exists
				$this->tail = $vowel->tail;
			}
		}
		else
		{
			// When vowels not exist
			$this->head = $consonant->head;
			$this->tail = $consonant->tail;
		}
	}
	public static
	function main($args)
	{
		$sll = new SingleLL();
		// Constructed linked list
		$sll->addNode('R');
		$sll->addNode('E');
		$sll->addNode('G');
		$sll->addNode('U');
		$sll->addNode('L');
		$sll->addNode('A');
		$sll->addNode('T');
		$sll->addNode('I');
		$sll->addNode('O');
		$sll->addNode('N');
		echo " Before Arrange\n";
		// R → E → G → U → L → A → T → I → O → N → NULL
		$sll->display();
		$sll->arrangeNodes();
		echo " After Arrange\n";
		// E → U → A → I → O → R → G → L → T → N → NULL
		$sll->display();
	}
}
SingleLL::main(array());

Output

 Before Arrange
 R → E → G → U → L → A → T → I → O → N → NULL
 After Arrange
 E → U → A → I → O → R → G → L → T → N → NULL