我的php登录界面总是显示我账号密码错误。。。我找不出错误 恳求各位大神帮助

界面php

<code> <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
<h1>Welcome</h1>
<?phpif (!$_POST["user"]){    echo"<center>";?><table border="1"><form action="login.php" method="post">
<tr><td colspan="2" align="center">用户登录</td></tr>
<tr>
<td>输入用户名:</td>
<td><input type="text" name="user"></td>
</tr>
<tr>
<td>输入密码：</td>
<td><input type="password" name="pass"></td>
</tr>
<tr><td colspan="2" align="center"><input type="submit" value="登录"></td></tr>
</form></table>
<?php }else{    include"config.php";    $user=$_POST["user"];    $user=$_POST["pass"];    $sql="SELECT COUNT(id) FROM $db_examsystem_user WHERE name='$user' AND pass='$pass'";    $result=mysql_query($sql,$my_con);    $num=mysql_fetch_row($result);    if($num[0]==0)    {        echo"用户名或者密码错误，请检查！<p>";        echo"单击<a href="login.php">这里</a>重新登录<br>";        echo"单击<a href="regist.php">这里</a>进行注册";    }    else    {        setcookie("user",$user);        echo"登陆成功<p>";        echo"单机<a href="index.php">这里</a>进入系统";    }}?></p></code>