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>";     } }