求助php中类的写法，目前报错：Fatal error: Cannot access empty property in /home/safewolf/www/

Fatal error: Cannot access empty property in /home/safewolf/www/_TestCode/testClass.php on line 6

$this-> $class2   =   new   TestClass2(); 
改为$this-> class2   =   new   TestClass2(); 

$this->后的变量不带$符
/////////////////////////////////////////////////////////
class HttpFetch
{

    public $debug = false;
   
    public $refer ='';
}

$Fetch ->$refer='Referer....‘改为
$Fetch ->refer='Referer....‘