?php
class demo{
  public $name;
  public static $count=0;
  private function __construct($name){
    echo "create $name br/>";
    $this->name = $name;
    self::$count++;
  }
  public function __destruct(){
    echo "destory ".$this->name."br/>";
    self::$count--;
  }
  public static function create($name){
    if(self::$count>2){
      die("you can only create at most 2 objects.");
    }else{
      return new self($name);
    }
  }
}
$one = demo::create("one");
$two = demo::create("two");
$two = null;
$three = demo::create("three");