不加载非对象的结果

Question

我正在尝试使用列名'Teachername'获取MySQL表的结果发生了什么，当我搜索名称时，我收到一个错误，指出我试图获取非对象的属性

这是代码..

if (isset($_GET['teacher'])) {

 $teachername = $_GET['teacher'];

 $sql = "SELECT * FROM assignmenthandout WHERE teachername = $teachername";

  $result = $conn->query($sql);

  if ($result->num_rows > 0) {
    while($row = mysqli_fetch_array ($result)) {
      echo $row["assignmentname"]."<br />";
    }
  } else {
    echo "There are no assignments due";
  }

错误所在的行是

if ($result->num_rows > 0) {

谢谢！