关于PHP-数据库的 数据读取，Trying to get property 'num_rows' of non-object？

$sql = "SELECT id, firstname, lastname FROM MyGuests";
$result = $conn->query($sql);

if ( $result->num_rows > 0 ){
//输出数据
while ($row = $result->fetch_assoc() ){
echo "id:".$row["id"]."-Name:".$row["firstname"]."-lastname:".$row["lastname"]."
";
}
} else {
echo "0结果";
}

if ( $result->num_rows > 0 ) 改成$result['num_rows']就好了

$conn 不存在啊

if ( $result->num_rows > 0 ) -----》if ( $result->num_rows() > 0 )