帮忙检查错在哪里啊？？

<?php
include("opendata.php");$sqll="select * from admin where username='".$user."' and password='".$pass."'";
$a=Mysql_query($sqll) or (Mysql_error());if ($a){
setcookie('username',$user);
echo " 登录成功!<br><a href=admin_article_show.php>返回管理页面</a>";
}
else
{
echo "错误！<br><a href=login.php>返回登录页面</a>";
}
?>帮忙检查错在哪里啊？？