login system using php and mysql [duplicate]

I made changes to the program but it still gives me error ... it says the following:

  

mysqli_query () expects at least 2 parameters, 1 given.

<?php
$username="username";
$pwd="pwd";
include ("database.php");
session_start();
    /*nesta linha----->*/ $res= mysqli_query("select username, pwd from user where username='$username' and pwd='$pwd'") or die(mysql_error());
    if(mysql_num_rows($res)==0)
    {
        $res['username'] = $username;
        $res['pwd'] = $pwd;
        header('PaginaIniciaDoutora1.php');

    }
else{
    unset ($_SESSION['username']);
    unset ($_SESSION['pwd']);
    header('Pagina.php');
}

?>