Mysql 错误:非对象的属性 - Mysql error:property of non-object

function checkEmail($email){
  $sql = "SELECT email FROM users WHERE email='".$email."'";
  $conn = new mysqli($this->servername, $this->username, $this->password, $this->dbname);
  $result = $conn->query($sql);
  if ($result->num_rows > 0) {
    echo "prs";
  } else {
    echo "0 results";
  }   
}

我有这个功能。当我运行时出现脚本错误：注意：正在尝试获取非对象的属性...在第 21 行（如果（$result->num_rows> 0））。怎么了？

查询的结果可能为空检查结果

if(isset($result) && !empty($result)){
  if ($result->num_rows > 0)
  { 
    echo "prs"; 
  }
 else
 { 
   echo "0 results"; 
  }
} 
else
 echo('0 results');