PHP在线服务器中的会话验证问题

我在登录表单验证后设置了$_SESSION['admin']=1
之后，页面被重定向并检查$_SESSION['admin']值和加载内容
它在当地做得很好。但它不能在在线linux服务器上运行。有什么帮助吗。。？

admin.php

<?php
   session_start();
   $admin = 0;
   if(isset($_SESSION['admin'])){
      $admin = $_SESSION['admin'];
   }
   if($admin == 0){
?>
<form action="" class="login">
     <label>User Name :</label>
     <input type="text" class="uname"/>
     <label>Password :</label>
     <input type="password" class="pwd"/>
     <input type="submit" class="lSubmit" value="SUBMIT"/>
     <p class="alert lAlert">test alert</p>
</form>
<?php }elseif($admin == 1){ ?>
     <h1>Welcome Site Admin..!!</h1>
<?php } ?>

JQuery

$('.lSubmit').click(function(){
    var name = $('.uname').val();
    var pwd = $('.pwd').val();
    $.post("validation/login.php",{name:name,pwd:pwd}).success(function(data){
        var obj = jQuery.parseJSON(data);
        if(obj.success == 1){
            $('.alert').css('color','#067800');
            window.location = "/admin.php";
        }else{
            $('.alert').css('color','#CC0000');
        }
        $('.lAlert').text(obj.msg);
        $('.lAlert').fadeIn('slow');
    });
    return false;
});

验证/登录.php

<?php
 session_start();
 $name = $_POST['name'];
 $pwd = $_POST['pwd'];
 $err['success'] = 0;
 $err['msg'] = '';
 if($name == ''){
  $err['msg'] = 'Name required';
 }else if($pwd == ''){
  $err['msg'] = 'Password required';
 }else if($name != 'admin'){
  $err['msg'] = 'Wrong username';
 }else if($pwd != 'admin'){
      $err['msg'] = 'Wrong password';
 }else{
  $err['msg'] = 'Success';
  $err['success'] = 1;
  $_SESSION['admin'] = 1;
 }
 echo json_encode($err);
?>

当我运行此程序时，它已成功进行验证，如果用户名和密码为admin，则它在警报中显示Success。但在它被重定向后，表单再次只加载到在线服务器中。但在当地，欢迎文字已经加载。但在网上的其他页面上，会话运行良好。有人能帮我吗？