一个小登陆，总是提示我用户名或密码错误

Question

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


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