“如何修复”此函数在MongoDB-PHP中调用getMatchedCount（）

Question

这是我用来检查用户名可用性的编码。我认为getMatchedCount()方法/函数有问题。这是因为，我在那一行不断出现错误，错误是：

'Fatal error: Uncaught Error: Call to a member function getMatchedCount() on null in C:\xampp\htdocs\project\OrgChart-master\OrgChart-master\memberForgotPassword.php:35 Stack trace: #0 {main} thrown in C:\xampp\htdocs\project\OrgChart-master\OrgChart-master\memberForgotPassword.php on line 35'

如何解决此问题？还是有其他方法可以从mongodb数据库中检查用户名的可用性？

预先，谢谢您对我的帮助。 ：）

if(isset($_POST['registration']))
{
       $registration = ($_POST['registration']);

       if(!empty($registration))
       {
            //$cursor = $collection->find(array("registration" => $registration));
            //$result = $cursor->count();

            $result = $db->collection1->findOne(array("registration"=>$registration));
            if($result->getMatchedCount()==0)
            {
                echo "Username Available";
            }
            else
            {
                echo "Please Enter The Correct Username";
            }

       }
 }