无法增加静态类变量PHP

我创建了一个接口类SellableItems，它是由tv和tennisBall两个产品实现的。两者都包含静态变量$count，我需要在递增和递减之后执行该变量。但是有一个StoreFactory类负责用静态工厂成员函数初始化对象。

问题:

当我测试它对下面给出的代码，我得到一个语法错误:* *

解析错误:语法错误，意外的'++' (T_INC) inD: ' xammp '根' ' ' inheritence ' FactoryPattern.php oop实践练习第36行

<?php
 interface SellableItems{
    public function addItem();
    public function removeItem();
 }
 class tennisBall implements SellableItems{
    public static $count=0;
    public function addItem(){
        return self::$count++;
    }
    public function removeItem(){
        if(self::count>0){
        return self::count--;
        }
        else {
            echo "<br/>Sorry, Stock empty";
        }
    }
    public function ShowData(){
        echo "<br/>There are ".self::$count." ".__CLASS__;
    }
 }

 class tv implements SellableItems{
    static $count=0;
    public function addItem(){
        return self::count++;
    }
    public function removeItem(){
        if(self::count>0){
        return self::count--;
        }
        else {
            echo "<br/>Sorry, Stock empty";
        }
    }
    public function ShowData(){
        echo "<br/>There are ".self::$count." ".__CLASS__;
    }
 }

 class StoreFactory{
    public static function factory($item){
        switch($item){
            case "tennisBall":
                $product=new tennisBall();
            break;
            case "tv":
                $product=new tv();
            break;
            default:
            die("<br/>WRONG Choice OF PRODUCT: {$item}");
        }
        if($product instanceof SellableItems){
            return $product;
        }
        else{
            die("</br>Sory cannot create particular Product");
        }
    }

 }
$Instance=StoreFactory::factory("tennisBall");
$Instance->addItem();
$Instance->addItem();
$Instance->ShowData();
?>