建站大佬来帮忙，网站后台进不去

网上找了一个网站的源码，建好以后首页没问题就是后台进不去，输入账号密码后提示登录成功，但是点击确定后又回到了登录界面，有人说是 PHP 版本的问题，它支持 5.2-5.5，我都挨个试过了，还是不行。是 login.php 的问题吗？代码我也贴上来，大佬给看看。我看到里面有验证码的代码，我在登录的时候没有看到验证码。

============================================

<?php

/**

* 登录

**/

$verifycode = 0;//验证码开关

if(!function_exists("imagecreate") || !file_exists('code.php'))$verifycode=0;

include '../ayangw/common.php';

if(isset($_POST['user']) && isset($_POST['pass'])){

$user=daddslashes($_POST['user']);

$pass=daddslashes($_POST['pass']);

$code=daddslashes($_POST['code']);

$pass = md5($pass.$password_hash);

if ($verifycode==1 && (!$code || strtolower($code) != $_SESSION['vc_code'])) {

unset($_SESSION['vc_code']);

@header('Content-Type: text/html; charset=UTF-8');

exit("<script language='javascript'>alert('验证码错误！');history.go(-1);</script>");

}elseif($user == $conf['admin'] && $pass== $conf['pwd']) {

unset($_SESSION['vc_code']);

$session=md5($user.$pass.$password_hash);

$token=authcode("{$user}\t{$session}", 'ENCODE', SYS_KEY);

setcookie("admin_token", $token, time() + 604800);

wsyslog("登陆后台成功!","登陆 IP:".real_ip().",城市:".get_ip_city(real_ip()));

@header('Content-Type: text/html; charset=UTF-8');

exit("<script language='javascript'>alert('登陆管理中心成功！');window.location.href='./';</script>");

}else {

unset($_SESSION['vc_code']);

@header('Content-Type: text/html; charset=UTF-8');

exit("<script language='javascript'>alert('用户名或密码不正确！');history.go(-1);</script>");

}

}elseif(isset($_GET['logout'])){

setcookie("admin_token", "", time() - 604800);

@header('Content-Type: text/html; charset=UTF-8');

exit("<script language='javascript'>alert('您已成功注销本次登陆！');window.location.href='./login.php';</script>");

}elseif($islogin==5){

@header('Content-Type: text/html; charset=UTF-8');

exit("<script language='javascript'>alert('您已登陆！');window.location.href='./';</script>");

}

$title='用户登录';

?>

<?php

@header('Content-Type: text/html; charset=UTF-8');

?>

<!DOCTYPE html>

<html>

<head>

<meta charset="utf-8"/>

<meta name="viewport" content="width=device-width, initial-scale=1"/>

<title><?php echo $title ?></title>

<link href="//lib.baomitu.com/twitter-bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>

<link href="//lib.ayangw.com/nifty/nifty.min.css" rel="stylesheet">

<script src="//lib.baomitu.com/jquery/1.12.4/jquery.min.js"></script>

<script src="//lib.baomitu.com/twitter-bootstrap/3.3.7/js/bootstrap.min.js"></script>



</head>

<div style="padding-top:70px;">

<div class="col-xs-12 col-sm-10 col-md-8 col-lg-6 center-block" style="float: none;">

<div class="panel panel-primary">

<div><h3>管理员登陆</h3></div>

<div>

<form action="./login.php" method="post" role="form">

<div>

<span><span class="glyphicon glyphicon-user"></span></span>

<input type="text" name="user" value="<?php echo @$_POST['user'];?>" placeholder="用户名" required="required"/>

</div><br/>

<div>

<span><span class="glyphicon glyphicon-lock"></span></span>

<input type="password" name="pass" placeholder="密码" required="required"/>

</div><br/>

<?php if($verifycode==1){?>

<div>

<span><span class="glyphicon glyphicon-adjust"></span></span>

<input type="text" class="form-control input-lg" name="code" placeholder="输入验证码" autocomplete="off" required>

<span style="padding: 0">

<img src="./code.php?r=<?php echo time();?>"height="35"onclick="this.src='./code.php?r='+Math.random();" title="点击更换验证码">

</span>

</div><br/>

<?php }?>

<div>

<div><input type="submit" value="登陆" class="btn btn-primary form-control"/></div>

</div>

</form>

</div>

</div>

</div>

</div>