php 页面跳转后session为空

<?php session_start();if($_GET['action'] == "logout"){ unset($_SESSION['username']); unset($_SESSION['userid']); exit('注销成功！<a href="javascript:history.back(-1);">点击返回</a>'); } if(isset($_POST['submit']) == ""){ exit('非法访问！！'); } $username= htmlspecialchars(trim($_POST['username']));$pwd = MD5(trim($_POST['pwd']));include('conn.php');$mql="select userid from users where username ='$username' and pwd='$pwd' limit 1";  $check_user =mysql_query($mql,$conn);  if($result =mysql_fetch_array($check_user)){ $_SESSION['username']= $username; $_SESSION['userid'] = $result['$userid'];  echo $username.'，欢迎到来！<a href="my.php">用户中心</a><br/>'; echo '<a href="index.php">点击进入主页</a>'; exit();}else{ echo '登陆失败！点击<a href="javascript:history.back(-1);">返回</a>'; } ?>

<?php session_start();if($_GET['action'] == "logout"){	unset($_SESSION['username']);	unset($_SESSION['userid']);	exit('注销成功！<a href="javascript:history.back(-1);">点击返回</a>');	}if(isset($_POST['submit']) == ""){	exit('非法访问！！');	}$username= htmlspecialchars(trim($_POST['username']));$pwd = MD5(trim($_POST['pwd']));include('conn.php');$mql="select userid from users where username ='$username' and pwd='$pwd' limit 1";  $check_user =mysql_query($mql,$conn);  if($result =mysql_fetch_array($check_user)){	$_SESSION['username']= $username;	$_SESSION['userid'] = $result['$userid'];		echo $username.'，欢迎到来！<a href="my.php">用户中心</a><br/>';	echo '<a href="index.php">点击进入主页</a>';	exit();}else{	echo '登陆失败！点击<a href="javascript:history.back(-1);">返回</a>';	}?>

<?php session_start();var_dump($_SESSION['userid']);if(! isset($_SESSION['userid'])){ echo "<a href='login.php'>登陆</a> <a href='zhuce.html'>注册</a>";}else{ 	$username =$_SESSION['username'];	echo $username."，欢迎到来<a href='loginok.php?action=logout'>注销</a>";	echo $_SESSION['userid']; 	echo "<a href='login.php'>登陆</a> <a href='zhuce.html'>注册</a>";}