Question

提示错误：Notice: Undefined variable: _SESSION in C:\wamp\www\1\top.php on line 50

50句：if($_SESSION['username']!="")，帮忙改下

Answer 1

使用SESSION必须在页面顶端加上 

<?php session_start();?>

追问

加了，然后错误没了，可是出现Notice: A session had already been started - ignoring session_start() in C:\wamp\www\1\top.php on line 3
第3句session_start();

追答

代码给我看一下, QQ：84702448

Answer 2

if(!empty($_SESSION['username'])

<?php

ob_start;

session_start();?>

Answer 3

用isset呢?