Queue in PHP

<?php

class Queue {

   // Head, tail, and size of queue.
   private $head;
   private $tail;
   private $size;

   public function isEmpty() {
       return  $this->size <=0;
   }
   
   public function peek() {
       if($this->isEmpty()) {
           throw new Exception("Queue is empty!");
       }
       return $this->head->value;
   }

   public function enqueue($value) {
       if($this->isEmpty()) {
           $this->tail = new Node($value, null);
           $this->head = $this->tail;
           $this->size++;
       }
       else {
           $this->tail->next = new Node($value, null);
           $this->tail = $this->tail->next;
           $this->size++;
       }
   }

   public function dequeue() {
       if($this->isEmpty()) {
           throw new Exception("Queue is empty!");
       }
       if($this->size == 1) {
           $tmp = $this->head;
           $this->head = null;
           $this->tail = null;
           $this->size = 0;
           return $tmp->value;
       }
       else {
           $tmp = $this->head;
           $this->head = $this->head->next;
           $this->size--;
           return $tmp->value;
       }
   }

   public function poll() {
       if($this->isEmpty()) {
           return null;
       }
       if($this->size == 1) {
           $tmp = $this->head;
           $this->head = null;
           $this->tail = null;
           $this->size = 0;
           return $tmp->value;
       }
       else {
           $tmp = $this->head;
           $this->head = $this->head->next;
           $this->size--;
           return $tmp->value;
       }
   }

   public function size() {
       return $this->size;
   }

   public function __toString() {
       $res = "{";
       $tmp = $this->head;
       while($tmp) {
           $res .= (string) $tmp->value;
           if($tmp->next) {
               $res .= ", ";
           }
           $tmp = $tmp->next;
       }
       return $res;
   }
}

// Supporting Node class.
class Node {
   public $value;
   public $next;

   public function __construct($value, $next) {
       $this->value = $value;
       $this->next = $next;
   }
}