PHP数3退1対象向けの書き方

<?php
        //                      3 1
        class Monkey {
                /**
                 *       
                 *        $id    
                 *          $left              
                 *          $right             
                */
                        public $id;
                        public $left;
                        public $right;
        }
        class Circle{
                /**
                 *      
                 *           $ct    
                 *             $first 
                 *             $last  
                 */
                public $ct = 0;
                public $first;
                public $last;
                //       
                //        
                function __construct($c){
                        for($i=0;$i<$c;$i++){
                                $this->add();
                        }
                }

                //       
                function add(){
                        $m = new Monkey();
                        $m->id = $this->ct;
                        if($this->ct==0){
                                $this->first = $m;
                                $this->last = $m;
                                $m->left = $m;
                                $m->right = $m;
                        }else{
                                $this->last->right = $m;
                                $m->left = $this->last;
                                $m->right = $this->first;
                                $this->first->left = $m;
                                $this->last = $m;
                        }
                        $this->ct++;
                }

                //       
                function del($m){
                        if($this->ct <=0){
                                return;
                        }else if($this->ct == 1){
                        
                                $this->first = $this->last = null;
                        }else{
                                $m->left->right = $m->right;
                                $m->right->left = $m->left;

                                if($m == $this->first){
                                        $this->first = $m->right;
                                }else if($m == $this->last){
                                        $this->last = $m->left;
                                }
                        }
                        $this->ct--;
                }
        
        }
        /*
                          500         
                                          
        */
        $circle = new Circle(500);
        $number = 0;
        $mk = $circle->first;
        while($circle->ct > 1){
                $number++;
                if($number == 3){
                        $number = 0;
                        $circle->del($mk);
                }
                $mk = $mk->right;
        }
        echo $circle->first->id;
?>