Online PHP editor | output for CdaQT

<?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 " Sorry, Stock empty"; } } public function ShowData(){ echo " There are ".self::$count." ".__CLASS__; } } class tv implements SellableItems{ static $count=0; public function addItem(){ return self::count++; } public function removeItem(){ if(self::count>0){ return self::count--; } else { echo " Sorry, Stock empty"; } } public function ShowData(){ echo " There are ".self::$count." ".__CLASS__; } } class StoreFactory{ public static function factory($item){ switch($item){ case "tennisBall": $product=new tennisBall(); break; case "tv": $product=new tv(); break; default: die(" WRONG Choice OF PRODUCT: {$item}"); } if($product instanceof SellableItems){ return $product; } else{ die("Sory cannot create particular Product"); } } } $Instance=StoreFactory::factory("tennisBall"); $Instance->addItem(); $Instance->addItem(); $Instance->ShowData();