用户名或密码错误html,一个小登陆，总是提示我用户名或密码错误

$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 "";

}

else

{

echo "";

}

}