php中Notice Trying to get property of non-object

出现这个问题的原因会有很多种情况,我的是下面这一种:

原本数据库语句我是这么写的:

$sql = "SELECT * FROM users WHERE u_name =".$name;

改成这样就没报错了:

$sql = "SELECT * FROM users WHERE u_name ='".$name."'";