为什么总显示用户名密码错误

Question

$conn=mysql_connect('localhost','root','root');
mysql_select_db('testdb',$conn);
mysql_query('set name utf8',$conn);


if (!empty($_REQUEST['n'])||!empty($_REQUEST['p']))
{
    $username=$_REQUEST['n'];
    $password=$_REQUEST['p'];
    $sql="select * from admin where username='$username'and password='$password',$conn";
    $re=mysql_query($sql,$conn);
    $arr=mysql_fetch_array($re);
    if ($arr)
    {

        echo "<script type=text/javascript>alert('登录成功');window.location.href='login-1.php';</script>";
    }
    else
    {

        echo "<script type=text/javascript>alert('用户名或密码错误');/*window.location.href='login-second.html';*/</script>";
    }
}

并且Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean givenin F:\lx\lx\login-1.php on line 19