刚学php,问个傻瓜级问题

<?php
/* public: 数据库连接参数 */
  $Host    = "localhost";
  $DB = "guipao";
  $User    = "root";
  $Password = "123456";
 
 
/* 建立连接、选择数据库*/
$conn=mysql_connect($Host, $User, $Password, $DB);

$code=$_POST["checkcode"];  //验证码
$user=$_POST["user"];        //用户名
$Pass=$_POST["Pass"];        //密码

//判断验证码
Session_start();
if($code!=$_SESSION["Checknum"])
{
echo " <script>alert('验证码不正确');history.go(-1); </script>";
exit(0);
}

//连接数据库，判断用户名和密码是否正确
$conn=mysql_connect($Host, $User, $Password, $DB) or print(mysql_error());
echo $conn;
echo " <br>";
$sql="select * from adminlist where adminname='".$user."' and psw='".$Pass."' ";
echo $sql;
$rs=mysql_query($sql) or print(mysql_error());;
echo $rs;
?>


mysql_query的错误提示：No database selected

数据库连接是通的输出连接ID Resource id #4
sql语句是正确的，select * from adminlist where adminname='admin' and psw='admin'

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$conn=mysql_conn