where clause in PHP /mysql

[dot_net_help]

how can i pass vraiable in mysql i have folling code

                          $username = $_REQUEST['username'];
		$password = $_REQUEST['password'];

$result = mysql_query("SELECT * FROM login_db where LoginID= 'demo' and loginpassword1= 'demo'  ");



I neeed to pas values in variables   $username  and $password 

please corrrect above where clause